P0688
ECM/PCM Power Relay Sense Circuit OpenP0688 is a generic OBD-II powertrain diagnostic trouble code: ECM/PCM Power Relay Sense Circuit Open. It is logged by the engine control unit when the glow monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.
What P0688 means
Most vehicles feed switched battery power to the engine control module and its main loads through a single relay, variously called the main relay, ECM power relay or system relay. As well as powering the module itself, that relay commonly supplies fuel injectors, ignition coils, sensors and other engine-management loads. So the module can confirm that the relay actually closed when commanded and opened when released, one of the relay's outputs is routed back to a dedicated sense, or feedback, input on the module.
P0688 sets when the module commands the relay on but sees no voltage returning on that sense input, or sees the feedback disappear while the relay should still be energised, for a calibrated period. It is an open-circuit fault report on the feedback path rather than on the relay coil drive itself. The same sense line is also how many modules know when power has genuinely been removed after key-off, which matters because they stay awake briefly to write learned values and park actuators.
Consequences range widely. If the relay really is failing to close, the vehicle will crank without starting or will stall as soon as it fires, because injectors and coils lose power. If the relay is working and only the feedback wire is broken, the engine may run perfectly and the code appears with no drivability complaint at all, sometimes showing up overnight.
Common causes
Most-frequently reported root causes when P0688 is logged.
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1
Failed main or ECM power relay with burnt, pitted or intermittent contacts.
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2
Corroded, spread or loose relay socket terminals, or a relay that is not fully seated.
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3
Open, chafed or broken wiring in the sense feedback line back to the module.
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4
Blown fuse or fusible link on the relay's switched output circuit.
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5
Voltage drop or corrosion in the battery feed and grounds serving the relay and module.
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6
Water intrusion into the underhood fuse and relay box bridging or corroding terminals.
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7
Aftermarket wiring, alarms, remote starters or accessories spliced into the relay circuit.
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8
Internal failure of the module's sense input, which is the least likely cause and should be confirmed last.
Symptoms drivers notice
How to diagnose P0688
A typical diagnostic flow when this code is present.
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1
Read all stored codes and freeze-frame data, and note which other circuits lost power at the same instant.
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2
Inspect the relay, its socket and the whole fuse box for corrosion, water, heat damage, pushed-out pins and evidence of previous repairs or spliced-in accessories.
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3
Check the fuses and fusible links both feeding and fed by the relay, and confirm battery terminals and engine and body grounds are clean and tight.
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4
With the key on, verify the module is commanding the relay and compare the voltage at the relay's switched output with the voltage arriving on the sense line at the module connector.
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5
Voltage-drop test across the relay contacts and along the sense wiring with the circuit loaded, rather than checking continuity on a dead circuit.
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6
Wiggle-test the harness and connectors while watching live data or a meter, since this fault is very often intermittent.
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7
Bench-test or substitute the relay only after wiring and grounds check out, and treat module replacement as a last resort once the sense input is proven dead with good voltage present at its pin.
Related powertrain codes
Frequently asked questions
Can I keep driving with P0688?
It is not safe to rely on. The relay in question feeds the engine control module and usually the injectors and coils, so a marginal relay or connection can stall the engine without warning at any speed. Get it diagnosed before making longer trips.
Why did the code appear when the car runs fine?
The fault may be only in the feedback wire that tells the module what the relay is doing, not in the power path itself. The relay still switches power, so the engine runs, but the module cannot confirm it and stores the code, often during the key-off shutdown routine.
Will it fail an emissions test?
Yes. The malfunction indicator lamp being on is an automatic fail on an OBD-II inspection, and the readiness monitors may also be incomplete if power has been interrupted.
Is this expensive to fix?
Usually not. Most cases come down to a relay, a corroded socket, a fuse or a broken wire, all inexpensive parts. The cost rises only if the harness needs significant repair or, rarely, if the control module has to be replaced and programmed.
Disabling P0688 in software
RaceTune can permanently disable P0688 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.
ECU families we can disable P0688 on
We hold the DaVinci A2L disable definitions for these families, so the exact P0688 path and mask addresses are mapped. verified marks a confirmed disable definition. We support many more — upload your file and our identifier will match it automatically.
- Bosch MD1CP002 verified
- Bosch MD1CS001 verified
Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.
Got P0688 in your scan?
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