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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 at a glance
CodeP0686
SystemEngine and transmission
GroupP06xx, Control unit and auxiliary outputs
TypeGeneric: 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.

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 this same explanation and whether it’s confirmed, pending or permanent.

With Free you read this code with its short description for nothing, and clear it if the engine stores it over standard OBD. With Pro you get the full explanation, how the car was running when it was logged (revs, temperature, load) and clearing in every control unit.

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 the ITV. It’s all in how to read and clear fault codes and in the ITV and OBD.

Similar codes

P0686 is a generic code, so it means the same on any make, for example on Alfa Romeo, Dacia, Kia, Nissan, Opel / Vauxhall, Škoda. See what Final OBD does on your car in makes and models.

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.