What does Mercury fault code 621-5 mean?

In current Mercury diagnostic publication 90-8M0237508 for the covered V6 and V8 platform, 621-5 means valid system voltage is lower than expected. Mercury’s operator action is to return to port, turn off unnecessary electrical loads, check the battery connections, and service the engine before its next use. That definition is family-specific.

Does Mercury fault code 621-25 always mean low voltage?

No. Do not use 621-25 as a universal Mercury low-voltage translation. The scoped V6/V8 table on this page uses 621-4 for high system voltage and 621-5 for low system voltage. A two-part code must be checked against the correct engine-family publication.

Can I clear a Mercury fault by closing the message?

No. Closing or acknowledging a display message is not a repair. A display history list, an active fault, and the diagnostic history stored by a supported engine module are separate records. Photograph the alert before dismissing or deleting anything.

Does every stored Mercury fault have freeze-frame data?

No. Mercury diagnostic tools can provide fault history and operating snapshots where supported, but the available record varies by module, engine family, software, and fault. Do not assume every history entry contains the same freeze-frame data.

What does Mercury fault code 3043-6 mean?

Mercury dealer information issued in May 2026 identifies 3043-6 as an electronic-shift overtravel condition on a limited single-engine V8 application with current calibration. It is not a universal V8 code and it is not a verdict to replace the shift actuator. Preserve the alert and have the serial number, rigging configuration, gearcase, and current PCM calibration checked by an authorized Mercury dealer.

Where do I find my exact Mercury owner manual?

Use Mercury Marine’s owner-manual lookup with the engine serial number. The serial number is the reliable route to the correct model-year and engine-family instructions.

What should I send with a Mercury service request?

If you can bring the boat to HBW in Gores Landing, send a clear photo of the complete code and message, the affected engine, serial number, engine hours, horn pattern, whether the alert is active or history, and what the boat was doing when it appeared. At hbw.wiki/service, choose “Various Boat / Motor Repairs (See Details),” put the record in Details, and use the photo upload for the display image. HBW does not diagnose fault codes remotely. If the boat cannot come here, take the same record to your local authorized Mercury dealer.

Mercury Outboard Fault Codes: Legacy VesselView and Modern UFC Lookup

Boater checking live Mercury engine data in the Mercury Marine app, with a Mercury FourStroke outboard on the transom.

Last reviewed: 2026-07-28

Mercury Premier Dealer·Gores Landing, ON·Quote builder available

Quick answer: Search the complete code, then match the result to the engine family. Single-number alerts may come from Mercury's legacy VesselView list. A two-part code such as 621-5 is a Universal Fault Code that must be checked against the correct family publication. This lookup separates the current covered V6/V8 table from the mixed-product legacy list.

If the display shows low oil pressure, oil-pump failure, an active overheat, fuel leakage or overflow, loss of steering, or a gear that does not match the control command, skip the lookup and follow the stop-now guidance below. If the table does not resolve the alert, take the photo and serial number to your local authorized Mercury dealer. If you can bring the boat to HBW in Gores Landing, submit that record at hbw.wiki/service so we can prepare for an in-shop diagnosis. HBW does not diagnose fault codes remotely.

This is the code-number companion to our Mercury beep and alarm guide. The horn tells you the alert class on supported systems; the screen identifies the actual fault. Both strategies vary by engine family and year.

Reference status: HBW last checked this lookup on July 24, 2026 against Mercury diagnostic publication 90-8M0237508, December 2025, current MercNET literature results, and Mercury's May 2026 V6/V8 fault-code update. The meaning and owner action are included here, so you do not need to find a diagnostic-manual page.

Before You Use Either Table

Photograph the whole alert first. Capture both numbers, all text, the affected engine, engine hours, and whether the display calls it active or history.
Use the serial number. Mercury fault support changes by PCM calibration, model, year, controls, and connected modules. The exact Owner's Manual remains the operator authority.
Treat the named circuit as a clue, not a parts order. A sensor code can be caused by the sensor, wiring, supply voltage, connector, or the condition being measured.
Do not combine the two tables. A single legacy ID and a modern two-part UFC are different systems.
Do not use a truck-code database. Use the product-specific Mercury table for the full pair shown on your display.
Do not order a part from the code name. The table identifies the monitored circuit or condition. Diagnosis still has to separate the component, wiring, connector, supply voltage, calibration, and the physical condition being measured.

Which Mercury Code Format Are You Seeing?

What the display shows	Start here	Important scope
Text such as Water in Fuel or a single number such as 23	Legacy VesselView table	Mercury 90-898279, March 2007; mixed product, calibration-dependent
Two numbers separated by a dash, such as 621-5	Modern V6/V8 UFC table	Mercury 90-8M0237508, December 2025, plus May 2026 dealer update; only the covered V6/V8 applications
Horn only, with no code display	Mercury beep and alarm guide	Horn patterns vary by family, year, and rigging
Plain-language alert in the Mercury Marine app	Record the exact text and any number	SmartCraft Connect Mobile shows live data and fault notifications; it does not replace serial-specific service data

Modern UFC Table: Current 3.4L V6 and 4.6L V8 Reference

This table is scoped to the fault-code section of Mercury diagnostic publication 90-8M0237508, December 2025. Its Models Covered section lists these as 2018-and-newer PCM 112 applications; the publication cover starts at serial number 2B529482:

V6: 150R, 175, 175 Pro XS, 200, 200 SeaPro, 200R, and 225
V8: 200 Pro XS; 225 Pro XS and SeaPro; 250, 250 Pro XS, 250 SeaPro, and 250R; 300, 300 Pro XS, 300 SeaPro, and 300R

Not every listed pair applies to every model, PCM calibration, midsection, gearcase, or control configuration. The last cell starts with Mercury's operator action, paraphrased into plain language. Any following sentence adds a safe first check named by Mercury or explains what still requires diagnosis. Dealer measurements, pinouts, software, adaptations, calibration, and repairs remain shop work.

Read the last column literally. Service soon means arrange service; it does not mean the named part is proven bad. Return to port now means operate only as needed for a safe return, then service the engine before using it again. A stop instruction takes priority over both.

UFC pair(s)	Documented component or condition	What to do now
101-16 / 102-16 / 103-16 / 104-16	Ignition circuits 1-4: PCM output-control fault	Service engine soon. The code can involve the spark plug, plug lead, ignition coil, its connector or wiring; it does not prove the coil failed.
201-16 / 202-16 / 203-16 / 204-16 / 205-16 / 206-16 / 207-16 / 208-16	Fuel-injector circuits 1-8: PCM output-control fault	Service engine soon. Injector wiring, connections, fuel pressure and the injector itself require diagnosis before replacing parts.
301-24 / 301-25	Throttle-position sensor A input above or below valid limit	Service engine soon. Do not force the throttle; the sensor, connector, wiring and shared sensor supply require diagnosis.
302-24 / 302-25	Throttle-position sensor B input above or below valid limit	Service engine soon. Do not force the throttle; the sensor, connector, wiring and shared sensor supply require diagnosis.
311-6	Throttle-position sensors A and B faulted	Return to port now and service the engine before its next use. Do not force the throttle or keep cycling the controls.
331-6	Throttle-position sensors A and B disagree	Return to port now and service the engine before its next use. Do not force the throttle or keep cycling the controls.
341-6 / 342-6	Throttle-position sensor A or B adaptation fault	Service engine soon. The sensor adaptation, throttle mechanism, connector and wiring require diagnosis.
401-24 / 401-25	Time-sampled manifold-pressure input above or below valid limit	Service engine soon. The pressure sensor, connector, wiring, sensor supply and intake system require diagnosis.
402-24 / 402-25	Angle-sampled manifold-pressure input above or below valid limit	Service engine soon. The pressure sensor, connector, wiring, sensor supply and intake system require diagnosis.
404-6	Manifold-pressure and throttle-position sensor A signals disagree	Service engine soon. The pressure and throttle signals, their wiring and the intake system must be checked together.
405-6	Manifold-pressure and throttle-position sensor B signals disagree	Service engine soon. The pressure and throttle signals, their wiring and the intake system must be checked together.
407-17	Manifold-pressure barometric reading outside expected range	Service engine soon. The pressure sensor, wiring, connector and local barometric comparison require diagnosis.
431-21	Oil pressure below the specified threshold	Stop the engine and check the oil level. If the level is correct or the alert persists, return to port and service the engine before its next use.
431-24 / 431-25	Oil-pressure sensor input above or below valid limit	Service engine soon. Check the oil level first; a sensor-circuit code does not prove that actual oil pressure is safe.
511-24 / 511-25	Intake-air-temperature input above or below valid limit	Service engine soon. The temperature sensor, connector and wiring require diagnosis.
521-20	Starboard engine-coolant temperature above normal	Stop the engine and check the cooling-water inlet for blockage. If the alert persists, return to port and service the engine before its next use.
521-24 / 521-25	Starboard coolant-temperature sensor input above or below valid limit	Service engine soon. The sensor, connector and wiring require diagnosis; separately confirm the engine is not actually overheating.
531-24 / 531-25	Oil-temperature sensor input above or below valid limit	Service engine soon. Check the oil level; the temperature sensor, connector and wiring require diagnosis.
572-24 / 572-25	Bank 1 exhaust-gas-temperature sensor input above or below valid limit	Service engine soon. The exhaust-temperature sensor, connector and wiring require diagnosis. Do not ignore signs of actual overheating.
601-4 / 601-5	Sensor-supply voltage A higher or lower than expected	Return to port now and service the engine before its next use. A shorted sensor or fault in the shared supply wiring can affect several readings.
602-4 / 602-5	Sensor-supply voltage B higher or lower than expected	Return to port now and service the engine before its next use. A shorted sensor or fault in the shared supply wiring can affect several readings.
603-4† / 603-5†	Sensor-supply voltage C higher or lower than expected	Service engine soon. This circuit requires current Mercury calibration information and dealer diagnosis.
621-4	Valid system voltage higher than expected	Return to port now and service the engine before its next use. Charging-system output, connections and wiring require diagnosis.
621-5	Valid system voltage lower than expected	Return to port now. Turn off unnecessary electrical loads, check the battery connections, and service the engine before its next use.
711-6	Oil-level sensor fault	Check the oil level on the dipstick, then service the engine soon. Do not assume a sensor fault means the oil level is safe.
711-12	Oil-level sensor invalid or disabled under present conditions	Check the oil level on the dipstick, then service the engine soon.
711-19	Oil-level test could not run, so level could not be determined	Check the oil level before continuing to operate the engine.
711-24	Oil-level sensor input above valid limit	Check the oil level on the dipstick, then service the engine soon.
713-21	Crankcase oil level below the specified threshold	Check the oil level before continuing to operate the engine. Add only the approved oil and do not overfill.
821-1 / 821-27	Wideband oxygen-sensor output circuit open/high resistance or short/low resistance	Service engine soon. The oxygen sensor, connector and wiring require diagnosis.
822-4 / 822-5 / 822-16	Wideband oxygen-sensor heater high, low, or output-control fault	Service engine soon. For 822-16, check the O2-sensor fuse only if your owner manual identifies it; otherwise leave circuit diagnosis to a technician.
902-4 / 902-5	Port short-term fuel-trim correction higher or lower than expected	Service engine soon. Fuel pressure, air leaks, injectors, oxygen-sensor feedback and wiring are among the systems that require diagnosis.
1012-24 / 1012-25	Engine trim-position input above or below valid limit	Service engine soon. The trim display may be inaccurate; the sensor linkage, connector and wiring require diagnosis.
1021-24 / 1021-25	Shift-actuator position sensor A input above or below valid limit	Return to port now and service the engine before its next use. Do not force the control or rely on an uncertain gear indication.
1023-24 / 1023-25	Shift-actuator position sensor B input above or below valid limit	Return to port now and service the engine before its next use. Do not force the control or rely on an uncertain gear indication.
1024-6	Shift-actuator position sensors A and B disagree	Return to port now and service the engine before its next use. Do not force the control or rely on an uncertain gear indication.
1025-6	Both shift-actuator position sensors faulted	Return to port now and service the engine before its next use. Do not force the control or rely on an uncertain gear indication.
1051-23	Camshaft synchronization state fault	Service engine soon. Camshaft and crankshaft sensing, connections and wiring require diagnosis.
1052-6	Crankshaft or camshaft trigger fault	Service engine soon. Crankshaft and camshaft sensing, connections and wiring require diagnosis.
1061-24 / 1061-25	Mechanical throttle-demand sensor A input above or below valid limit	Service engine soon. Move the control gently; the demand sensor, connector, wiring and adaptation require diagnosis.
1062-24 / 1062-25	Mechanical throttle-demand sensor B input above or below valid limit	Service engine soon. Move the control gently; the demand sensor, connector, wiring and adaptation require diagnosis.
1063-24 / 1063-25	Mechanical shift-demand sensor A input above or below valid limit	Service engine soon. If the indicated gear does not match the control position, stop using the boat and arrange service.
1064-24 / 1064-25	Mechanical shift-demand sensor B input above or below valid limit	Service engine soon. If the indicated gear does not match the control position, stop using the boat and arrange service.
1071-6 / 1072-6	Mechanical throttle-demand sensor A or B adaptation fault	Service engine soon. The demand sensor adaptation, control mechanism, connector and wiring require diagnosis.
1073-6	Mechanical throttle-demand sensors A and B disagree	Return to port now and service the engine before its next use. Do not force or repeatedly cycle the control.
1074-6	Both mechanical throttle-demand sensors faulted	Return to port now and service the engine before its next use. Do not force or repeatedly cycle the control.
1077-6	Mechanical shift-demand sensors A and B disagree	Return to port now and service the engine before its next use. Stop if the indicated gear does not match the control position.
1078-6	Both mechanical shift-demand sensors faulted	Return to port now and service the engine before its next use. Stop if the indicated gear does not match the control position.
1108-25	Water detected in the fuel system	Service engine soon. Follow the water-in-fuel procedure in the serial-specific owner manual; stop immediately if fuel is leaking or overflowing.
1109-23	Emergency-stop circuit active	Check the emergency-stop lanyard. Turn the key off and restart once; if the alert returns, service the engine soon.
2011-23	Engine Guardian active and limiting power	Reduce engine speed. Look for the separate root alert that caused Guardian to limit power.
2021-23	Guardian active because of oil temperature	Reduce engine speed. If the alert persists, stop the engine and arrange service.
2032-23	Guardian active because of exhaust-system temperature	Reduce engine speed. If the alert persists, stop the engine and arrange service.
2051-23	Guardian active because of low oil pressure	Reduce engine speed. If the alert persists, stop the engine and check the oil level. Do not continue if the level is correct but the alert remains.
2081-23	Guardian active because of engine overheat	Reduce engine speed. If the alert persists, stop the engine and check the cooling-water inlet for blockage.
2091-23	Guardian active because of excessive engine speed	If the alert persists, lower excessive trim, stop the engine and check the propeller for damage or incorrect size.
2092-23	Engine speed above its neutral limit	Reduce engine speed and return the control to neutral. Service the engine if the alert repeats.
2101-23	Guardian active because the helm module requested a power limit	Reduce engine speed. Check the display for another alert or helm message that identifies the reason.
2111-23	Guardian active because of battery voltage	Reduce engine speed. Turn off unnecessary electrical loads and check the battery connections.
2124-20	Exhaust-manifold temperature above normal	Stop the engine and check the exhaust-water strainer for blockage. If the alert persists, return to port and service the engine before its next use.
3002-16	Active-exhaust-valve output-control fault	Service engine soon. The valve, connector, wiring and output circuit require diagnosis.
3012-6 / 3013-6 / 3014-6	Electronic throttle lost control, has an output fault, or is sticking	Return to port now and service the engine before its next use. Do not force or repeatedly cycle the throttle control.
3031-6† / 3032-6	Electronic shift commanded position differs from actual position, or control is lost	Return to port now and service the engine before its next use. Do not force the control; current software, calibration and shift hardware require dealer diagnosis.
3033-6† / 3034-6†	Electronic shift could not adapt in reverse or forward	Service engine soon. If the indicated gear does not match the control position, stop using the boat. Current software and calibration require dealer diagnosis.
3037-6	Electronic shift control timed out	Return to port now and service the engine before its next use. Do not force or repeatedly cycle the shift control.
3039-23	Reverse gear unavailable; a reverse command may produce forward gear	Return to port now and service the engine before its next use. Do not command reverse again; the boat may move forward.
3043-6†	Electronic shift actuator overtravel detected; dealer diagnosis required	If indicated gear does not match the control position, stop using the boat. Preserve the alert and have calibration, configuration, actuator travel and shift hardware checked by a dealer.
3049-16	Hydraulic-shift output-control fault	Return to port now and service the engine before its next use. Do not force or repeatedly cycle the shift control.
3061-16	Fuel-pump output-control fault	Return to port now and service the engine before its next use. Stop immediately if fuel is leaking or overflowing.
3152-16	Warning-horn output-control fault	Service engine soon. The horn may not sound reliably, so watch the display and do not depend on an audible warning.
3171-16	Starter output-control fault	Return to port now and service the engine before its next use. If the engine stops, a restart may not be available.
3181-16 / 3182-16	Trim-up or trim-down output-control fault	Service engine soon. Trim in the affected direction may not work; the switch, relay, wiring and output circuit require diagnosis.
4001-6 / 4002-6 / 4003-6	Demand, shift, or helm-module crosscheck fault	Return to port now and service the engine before its next use. Do not force or repeatedly cycle the controls.
4004-6	CAN X document 1 state-of-health fault	Return to port now and service the engine before its next use. Network power, wiring and connected modules require diagnosis.
4005-6	CAN P document 2 state-of-health fault	Service engine soon. Network power, wiring and connected modules require diagnosis.
4006-6	CAN P document 03 state-of-health fault	Return to port now and service the engine before its next use. Network power, wiring and connected modules require diagnosis.
4007-6	CAN X document 07 state-of-health fault	Return to port now and service the engine before its next use. Network power, wiring and connected modules require diagnosis.
4008-6	CAN X document 09 state-of-health fault	Return to port now and service the engine before its next use. Network power, wiring and connected modules require diagnosis.
4009-6	CAN X document 10 state-of-health fault	Return to port now and service the engine before its next use. Network power, wiring and connected modules require diagnosis.
4010-6	CAN X document 11 state-of-health fault	Cruise control may not work properly. Service the engine soon.
4011-23	CAN-bus authentication timeout	Service engine soon. Network power, wiring, module identity and current software require diagnosis.
4012-6	CAN P and CAN X state-of-health faults	Return to port now and service the engine before its next use. Network power, wiring and connected modules require diagnosis.
4013-23	Watchdog active	Return to port now and service the engine before its next use.
4014-6 / 4016-6	Watchdog-module or watchdog-data crosscheck fault	Return to port now and service the engine before its next use.
4501-23	Security device missing	Do not turn the key off. Return to port now and service the engine before its next use.
4502-23	Security system locked	Follow the unlock procedure in the serial-specific owner manual. If it cannot be unlocked normally, arrange service.
4503-23	Security system in setup mode	Follow the security setup instructions in the serial-specific owner manual or have an authorized dealer complete the setup.
4602-23	Low system voltage blocking other fault evaluation	Return to port now. Turn off unnecessary electrical loads, check the battery connections, and service the engine before its next use.

† Marked codes have May 2026 dealer-software and calibration guidance. See What Changed in May 2026 below.

The December 2025 manual contains 130 UFC pairs. Mercury's May 2026 dealer update adds 3043-6 for a limited single-engine V8 application, bringing this current scoped lookup to 131 searchable pairs. Do not apply this table to a different Mercury family.

What Changed in May 2026

The dagger-marked pairs have newer dealer guidance:

603-4, 603-5, 3031-6, 3033-6, and 3034-6 require assessment with current Mercury dealer software and PCM calibration information.
3043-6 is a newer electronic-shift overtravel code for a limited single-engine V8 configuration. It is not a universal V8 fault and it is not, by itself, a direction to replace the shift actuator.
On multiple-engine boats and jet conversions, whether 3043-6 should be active depends on configuration. That is a dealer-software check, not an owner reset.

This page publishes the owner-visible code and safe context. It does not reproduce Mercury's restricted dealer procedure, configuration credentials, or repair workflow.

What the UFC Suffix Means in This V6/V8 Publication

The number after the dash describes how the named component, value, or state failed. These short definitions are scoped to this publication; always interpret the complete pair.

Suffix	Meaning in the scoped table
1	Output circuit open or resistance too high
4	Valid value higher than expected
5	Valid value lower than expected
6	Device, calculation, adaptation, or process fault
12	Device invalid or disabled under present conditions
16	Output-control fault detected by the controller
17	Signal outside the expected range
19	Test could not run under present conditions
20	Temperature higher than normal
21	Value below a specified threshold
23	A documented state or condition exists
24	Sensor input above its valid limit
25	Sensor input below its valid limit
27	Input circuit shorted or resistance too low

That is why 621-5, not 621-25, is the low-system-voltage pair in this V6/V8 publication. The same suffix does not give you permission to invent a component meaning that is absent from the correct Mercury table.

What If Your Two-Part Code Is Not Here?

Do not force a match. Other Mercury families, including 40-60hp FourStroke, 75-115hp FourStroke, 150hp FourStroke, OptiMax, inline-four and inline-six Verado, V10 Verado, V12 Verado, Jet, and Racing applications, have their own service data, serial breaks, modules, and calibrations.

Record the complete pair, every word on the display, and the engine serial number. A match on the first half alone is not enough: 621-4 and 621-5 monitor the same system-voltage value but describe opposite conditions. If the full pair is absent, use the serial-number-specific publication or take the record to an authorized Mercury dealer.

Legacy VesselView Fault IDs: March 2007 Mixed-Product List

Mercury's VesselView operation manual 90-898279, March 2007, printed a single-number fault list through 247. Mercury's own note is the most important part: the displayed faults are determined by PCM calibration, not every fault is used on every engine, the information was accurate at the printing date, and it could change.

This is not an outboard-only master list. It mixes outboard, MerCruiser, DTS, diesel or transmission, drive-lube, hydraulic-shift, and legacy supercharged-Verado items. The grouped descriptions below preserve the display-level meaning without inventing cylinder assignments where the VesselView manual only repeats a generic label.

Legacy ID(s)	March 2007 display meaning	Scope note
1	Critical high battery voltage	Mixed-product VesselView
2	Critical low battery voltage	Mixed-product VesselView
3	Low cooling-water pressure	Application-dependent
4	Compressor overtemperature	Legacy compressor-equipped application
5-6	Critical throttle-position control fault	DTS/application-dependent
9	Engine Guardian active; power limited	Root fault appears separately
10-11	Knock sensor or knock-prevention fault	Application-dependent
12	Low engine-oil pressure	Four-stroke/inboard application as equipped
13	Remote two-stroke oil tank low	Oil-injected two-stroke
14	Engine-mounted two-stroke oil tank critically low	Oil-injected two-stroke
15-16	Manifold absolute-pressure sensor fault	Application-dependent
17	Excess air in idle state	Application-dependent
18	Oil-pump fault	Oil-injected two-stroke
19	Engine overspeed	Application-dependent
20-21	Engine overtemperature	Application-dependent
22	Warning-horn output fault	Application-dependent
23	Water in fuel	Application-dependent
24-35	Direct/air injector fault; twelve generic entries	Legacy DFI display list; no cylinder mapping in this table
36-37	Water-pressure sensor fault	Application-dependent
38	Break-in strategy remaining	Legacy PCM strategy
39-40	Engine-air-temperature sensor fault	Application-dependent
41-42	Air-compressor-temperature sensor fault	Compressor-equipped application
43-54	Ignition-coil fault; twelve generic entries	Cylinder mapping depends on service data
59-70	Fuel-injector fault; twelve generic entries	Cylinder mapping depends on service data
75-76	Fuel-level sender fault	Vessel/tank input
77	Crank/cam encoder fault	Application-dependent
78-79	Jack-plate position sensor fault	Boats equipped with jack plate input
80	Critical main-power-relay fault	Application-dependent
81	Main-power-relay backfeed	Application-dependent
82-83	Oil-tank level sensor fault	Oil-tank-equipped application
84-85	Engine-oil-pressure sensor fault	Application-dependent
89-90	Engine-oil-temperature sensor fault	Application-dependent
91	Engine-oil temperature high	Application-dependent
92	Paddle-wheel speed sensor fault	Vessel sensor
93-94	Pitot/speed-pressure sensor fault	Vessel sensor
95-96	Coolant-temperature sensor fault	Application-dependent
97-98	Trim-tab position sensor fault	Trim-tab-equipped vessel
99-100	Trim-tab controller fault	Trim-tab-equipped vessel
101	Critical SmartCraft communication fault	Network/application-dependent
102	SmartCraft communication fault	Network/application-dependent
103	Critical SmartCraft communication fault	Network/application-dependent
104-105	Seawater-temperature sensor fault	Vessel sensor
106-108	Critical shift-actuator fault	DTS/shift application
109	Shift-actuator fault	DTS/shift application
110	Shift-switch fault	Shift application
111-112	Coolant-temperature sensor fault	Application-dependent
113-114	Trim-tab sensor fault	Trim-tab-equipped vessel
115-116	Trim-tab control fault	Trim-tab-equipped vessel
117	Starter-solenoid/output fault	Application-dependent
118-119	Steering-angle sensor fault	Steering-equipped application
120-131	Throttle-position sensor fault; multiple channels	Application-dependent
132	Crankshaft-position sensor fault	Application-dependent
133	Critical low sensor-supply voltage	Application-dependent
134-135	Engine overspeed	Application-dependent
136-137	Boost-bypass valve fault	Legacy supercharged application
138	Engine-control-unit fault	Application-dependent
148	Shift-controller fault	DTS/shift application
149	Critical electronic-throttle-controller fault	Electronic throttle application
150	Critical SmartCraft communication fault	Network/application-dependent
151	Critical fuel-pump fault	Application-dependent
152	Idle-air-control valve fault	Application-dependent
153	Low drive-lube level	Sterndrive, not an outboard fault
154	Excessive calculated airflow	Application-dependent
155-157	Secondary manifold-pressure sensor fault	Application-dependent
158-160	Port exhaust-manifold temperature sensor fault or high temperature	Inboard/sterndrive application
163	Critical SmartCraft communication fault	Network/application-dependent
169-170	Engine-oil-pressure sensor fault	Application-dependent
171-172	Engine Guardian active; power limited	Root fault appears separately
173-174	Fuel pressure high or low	Application-dependent
175	Knock system not functioning	Application-dependent
176	Critical shift fault	Shift application
177-179	Starboard exhaust-manifold temperature sensor fault or high temperature	Inboard/sterndrive application
180-183	Throttle-position sensor fault or disagreement	Application-dependent
184-185	Trim relay fault	Trim-equipped application
186-190	Critical SmartCraft communication fault; multiple modules	Network/application-dependent
191-192	Tank 2 level sender fault	Vessel/tank input
193-194	Shift fault	Shift application
195	Thermostat fault	Application-dependent
196	Transmission overtemperature	Inboard/transmission application
197-199	Supercharger outlet-temperature sensor fault or high temperature	Legacy supercharged application
200	Boost-bypass valve fault	Legacy supercharged application
201-203	Engine-temperature sensor fault or overtemperature	Application-dependent
204-206	Block-temperature sensor fault or overtemperature	Application-dependent
207-208	Fuel-lift-pump fault	Application-dependent
209	Critical low engine-actuator voltage	Electronic-control application
210	Neutral overspeed	Electronic-control application
211	Reverse overspeed	Shift application
212	Start sequence aborted	Electronic-start application
213	Fuel overflow into vent system	Fuel-system application
214	Boost-bypass valve fault	Legacy supercharged application
215-216	Critical SmartCraft communication fault	Network/application-dependent
217	Low sensor-supply voltage	Application-dependent
218	Critical SmartCraft communication fault	Network/application-dependent
219	Shift command fault; control requested to neutral	DTS/shift application
220	Fuel overflow into vent system	Fuel-system application
221	Fuel-lift-pump float-switch fault	Fuel-system application
222	Fuel-lift-pump fault	Fuel-system application
223-225	Piston-cooling oil-jet pressure or sensor fault	Application-dependent
226	Critical SmartCraft communication fault	Network/application-dependent
227-232	Shift-pressure sensor fault	Hydraulic-shift application
233	Shift-system fault	Shift application
234	Shift pressure high	Hydraulic-shift application
235	Shift fault; control requested to neutral	Shift application
236	Emergency-stop circuit active	Application-dependent
237	Power limited by knock system	Guardian response
238	Overspeed in trailer position	Trim/engine application
239	Underwater impact detected	Application-dependent
240	Critical low battery voltage	Mixed-product VesselView
241	Charge-air temperature high	Legacy supercharged application
242	Fuel-purge-valve fault	Application-dependent
243-244	Trim-tab sensor fault	Trim-tab-equipped vessel
245	Shift-switch fault	Shift application
246	Power limited by supercharger overtemperature	Legacy supercharged application
247	Power limited by knock system	Guardian response

If your single-number code is absent, do not fill a gap with a neighbouring meaning. Mercury left gaps in this printed list, and connected modules can generate alerts outside it.

Stop-Now Conditions

The code table identifies the alert. What you observe determines the safest immediate response.

Low Oil Pressure

Stop the engine. Check the oil level using the engine position and procedure in the serial-number-specific manual. If the level is correct or the warning persists, do not continue running. A sensor-circuit alert does not prove that actual oil pressure is safe.

Two-Stroke Oil-Pump Fault

Stop and do not restart. Mercury's SC1000 documentation warns that an electrical oil-pump fault means lubricating oil is not being supplied.

Fuel Leak or Fuel-Overflow Warning

Stop the engine and do not restart it. Keep smoking, flames, sparks, and other ignition sources away. Any sign of fuel leakage requires repair before further operation. Do not try to “head in” under engine power.

Overheat or No Cooling-Water Flow

Reduce throttle immediately. When safe, shift to neutral and stop the engine. Inspect the cooling-water inlets only with the engine off. If water flow was absent or intermittent, or the warning remains after the checks allowed by your manual, do not continue running.

DTS, Shift, Steering, or CAN Fault

Reduce power and follow the exact display instruction. Do not force the control or linkage, and do not assume neutral or reverse remains available. If indicated gear does not match the command, neutral cannot be confirmed, steering is affected, or a critical communication alert persists, stop as soon as safe and arrange assistance. Any restart procedure must be specific to that engine, control system, and fault.

Voltage Warning

Turn off nonessential electrical loads and keep control of the boat. Touch battery connections only after key-off and electrical isolation. Do not universally raise RPM: some current systems can increase idle automatically during low voltage. If the alert is critical, Guardian is active, or throttle, shift, steering, or CAN operation is affected, stop safely and arrange assistance.

Active Fault, Display History, and Module History Are Different

An active fault is present now. A display history is the alert list stored by that display. A supported engine module can also hold diagnostic fault history and operating snapshots, but the available record varies by module and fault.

Photograph the exact code and text, affected engine, hours, horn pattern, and operating conditions before dismissing or deleting anything. Clearing a VesselView display's archived history clears that display list. It must not be described as erasing the PCM record a technician may read with CDS G3.

Likewise, do not assume every stored fault includes the same freeze-frame data or that every module holds the same number of entries. Mercury's current CDS G3 material describes fault history and operating snapshots only where supported.

How to Read the Alert on Common Mercury Displays

VesselView 403

Open the alert details and alarm-history screens using the controls shown on your display. An active fault is not repaired by closing its message; do not generalize one VesselView generation's clearing behaviour to another.

VesselView 704

Open the alert details and history through the display's Alerts screens. Expand the message for its description, severity, and code. Closing the window is not an engine-module history clear.

SC1000 System Tach

The descriptive fault view shows location, component, detail, corrective action, and additional faults through MORE, ACTION, and NEXT on supported systems. Do not use Master Reset to chase an engine fault; it restores gauge defaults and erases gauge setup and calibration.

SmartCraft Connect Mobile and the Mercury Marine App

For fault work, the current app provides live engine data and fault notifications. Older VesselView Mobile hardware can also work with the current Mercury Marine app. The phone view does not replace CDS G3 module tests, adaptations, configuration, or serial-specific service information. See our SmartCraft Connect Mobile guide.

Dealer CDS G3

CDS G3 provides the supported module's diagnostic fault history, operating snapshots where available, state-of-health information, tests, configuration, and programming functions. It is the shop tool, not a reason to guess at a repair from a public table.

What Helps the Shop Start Faster

If you can bring the boat to HBW in Gores Landing, collect the following before submitting a service request. On the existing form, choose Various Boat / Motor Repairs (See Details), put this record in Details, and use the photo upload for the display image. If the boat cannot come here, take the same record to your local authorized Mercury dealer.

A clear photo showing the complete number and all display text.
The engine serial number and which engine is affected on a multi-engine boat.
Engine hours and whether the display says active or history.
The horn pattern, including duration and whether it repeated.
What the boat was doing: key-on, idle, accelerating, cruising, shifting, trimming, or restarting.
Any recent battery, rigging, control, fuel-system, or service work.
Whether Guardian limited power, water flow changed, steering changed, or indicated gear disagreed with the control.

Do not clear or repeatedly restart just to make the message disappear. Preserve the evidence. For an in-shop diagnosis at HBW, submit the record at hbw.wiki/service and bring the boat to Gores Landing.

From the Shop

A fault code names the circuit or condition the computer noticed. It does not name the part to buy. The cleanest diagnostic starts with the full display photo, serial number, hours, battery voltage and what the boat was doing when the alert appeared.

Common Lookup Mistakes

Calling the March 2007 1-247 list a current or outboard-only master list.
Translating 621-25 as low voltage on every Mercury; the scoped V6/V8 publication uses 621-5.
Reading a sensor name as proof the sensor itself failed.
Treating Guardian as the root cause instead of the protective response.
Applying one VesselView menu path or clearing behaviour to every display generation.
Assuming closing a display alert clears the engine module's history.
Assuming every stored fault includes identical freeze-frame data.
Forcing a DTS control or trying a generic key cycle when gear or steering status is uncertain.

Sources

Mercury V6/V8 diagnostic publication 90-8M0237508, December 2025. Current dealer service literature for the covered 3.4L V6 and 4.6L V8 platform, starting serial 2B529482. HBW reviewed every listed fault meaning and operator action through MercNET on July 24, 2026.
Mercury Service Advisory 2026-06R2, May 2026. Current dealer information for V6/V8 shift and PCM fault-code updates. This article summarizes the owner-visible code changes without redistributing the restricted bulletin or its dealer procedure.
Mercury VesselView operation manual 90-898279, March 2007. Source of the mixed-product legacy display list. HBW reviewed the Mercury publication directly; the table above preserves its calibration and product-scope warnings.
Mercury SmartCraft Connect Mobile
Mercury Marine owner resources and serial-specific manual lookup

Frequently Asked Questions

What does Mercury fault code 621-5 mean?: In current Mercury diagnostic publication 90-8M0237508 for the covered V6 and V8 platform, 621-5 means valid system voltage is lower than expected. Mercury’s operator action is to return to port, turn off unnecessary electrical loads, check the battery connections, and service the engine before its next use. That definition is family-specific.
Does Mercury fault code 621-25 always mean low voltage?: No. Do not use 621-25 as a universal Mercury low-voltage translation. The scoped V6/V8 table on this page uses 621-4 for high system voltage and 621-5 for low system voltage. A two-part code must be checked against the correct engine-family publication.
Are the old VesselView numbers 1 through 247 all outboard codes?: No. Mercury manual 90-898279, dated March 2007, is a mixed-product VesselView display list. It includes outboard, MerCruiser, DTS, diesel or transmission, drive-lube, hydraulic-shift, and legacy supercharged-Verado items. Mercury also says the list is calibration-dependent, not every fault is used on every engine, and the information was current only at the time of printing.
Can I clear a Mercury fault by closing the message?: No. Closing or acknowledging a display message is not a repair. A display history list, an active fault, and the diagnostic history stored by a supported engine module are separate records. Photograph the alert before dismissing or deleting anything.
Does every stored Mercury fault have freeze-frame data?: No. Mercury diagnostic tools can provide fault history and operating snapshots where supported, but the available record varies by module, engine family, software, and fault. Do not assume every history entry contains the same freeze-frame data.
What does Mercury fault code 3043-6 mean?: Mercury dealer information issued in May 2026 identifies 3043-6 as an electronic-shift overtravel condition on a limited single-engine V8 application with current calibration. It is not a universal V8 code and it is not a verdict to replace the shift actuator. Preserve the alert and have the serial number, rigging configuration, gearcase, and current PCM calibration checked by an authorized Mercury dealer.
Where do I find my exact Mercury owner manual?: Use Mercury Marine’s owner-manual lookup with the engine serial number. The serial number is the reliable route to the correct model-year and engine-family instructions.
What should I send with a Mercury service request?: If you can bring the boat to HBW in Gores Landing, send a clear photo of the complete code and message, the affected engine, serial number, engine hours, horn pattern, whether the alert is active or history, and what the boat was doing when it appeared. At hbw.wiki/service, choose “Various Boat / Motor Repairs (See Details),” put the record in Details, and use the photo upload for the display image. HBW does not diagnose fault codes remotely. If the boat cannot come here, take the same record to your local authorized Mercury dealer.

Harris Boat Works · 5369 Harris Boat Works Rd, Gores Landing, ON · (905) 342-2153

See all Mercury outboard prices (CAD) · Repower guide · FAQ