P0686: ECM/PCM power relay control: circuit low
The ECU sees too low a voltage on the control of the relay that powers it, usually from a wire shorted to earth or a faulty relay.
| Code | P0686 |
|---|---|
| System | Engine and transmission |
| Group | P06xx, Control unit and auxiliary outputs |
| Type | Generic: it means the same on every make |
| Can you keep driving? | With care |
What P0686 means
The engine ECU is powered through a main relay that it switches on itself when the ignition is turned on and switches off a while after it is turned off. That relay usually feeds other engine components as well. This code means the ECU measures less voltage than expected on the relay control wire, which is typical of a wire touching earth or a relay with a damaged coil.
Most common causes
- Low battery or poor battery terminals
- Faulty main relay (shorted coil)
- Damp or corroded relay connector
- Chafed control wire touching the bodywork
- ECU fault (uncommon)
Symptoms
- Engine will not start or is slow to start
- Engine cuts out without warning while driving
- Engine warning light and other lights on
- Scanner unable to communicate with the ECU
Can you drive with P0686?
With care You can drive if the engine is running normally, but a power fault to the ECU can leave you stranded or stop the engine on the move. Have it checked soon.
Open the bonnetthe in-depth explanation
How to check it
Start with the battery and its terminals, and see whether the code returns after clearing it. A garage can temporarily swap the relay for an identical one from the same fuse box to see if the fault moves.
How to read and clear P0686 with Final OBD
Plug an OBD2 scanner in under the steering wheel, switch the ignition on and open Final OBD. Under Faults you’ll see P0686 with its description and whether it’s confirmed, pending or permanent.
If the engine stores it over standard OBD, with FREE you read this code with its explanation for nothing and clear it. With PRO you also read the faults of every control unit and clear them in all of them.
Clearing the code only turns the light off: if the cause is still there, it comes back. Clear it once it’s fixed, and not right before an inspection. It’s all in how to read and clear fault codes and in inspections and OBD.
Similar codes
- P0682 The cylinder 12 glow plug has an electrical fault
- P0683 The glow plug control unit at the engine control unit communication has an
- P0684 The glow plug control unit at the engine control unit communication gives
- P0685 ECM/PCM power relay control: circuit open
- P0687 ECM/PCM power relay control: circuit high
- P0688 ECM/PCM power relay sense: circuit open
- P0689 The engine control relay monitoring signal is low
- P068A The engine control relay switches off too early
- P0600 Serial communication link fault
- P0601 Internal control module memory checksum error
- P0602 Control module programming error
- P0603 Internal control module keep alive memory (KAM) error
What does code P0686 mean?
The engine ECU is powered through a main relay that it switches on itself when the ignition is turned on and switches off a while after it is turned off. That relay usually feeds other engine components as well. This code means the ECU measures less voltage than expected on the relay control wire, which is typical of a wire touching earth or a relay with a damaged coil.
Can you drive with P0686?
You can drive if the engine is running normally, but a power fault to the ECU can leave you stranded or stop the engine on the move. Have it checked soon.
How do you clear P0686?
With an OBD2 scanner and an app like Final OBD, which clears it for free if it’s stored by the engine control unit. But clearing it only turns the light off: if the cause isn’t fixed, P0686 comes back.
Read it on your car
Final OBD reads and clears engine faults through standard OBD for free, each one explained in plain English. With PRO it also reads the faults of every control unit, each make’s own codes included, and clears them in all of them.
