Skip to content
Language: EN
Download

P0685: ECM/PCM power relay control: circuit open

The ECU cannot operate the relay that powers it because of a break in the control circuit, which can stop the engine starting or make it cut out.

Code P0685 at a glance
CodeP0685
SystemEngine and transmission
GroupP06xx, Control unit and auxiliary outputs
TypeGeneric: it means the same on every make
Can you keep driving?With care

What P0685 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 control circuit for that relay is open: the command is not getting through or the relay is not responding.

Most common causes

  • Low battery or loose, corroded battery terminals
  • Blown main relay fuse
  • Faulty main relay or relay not seated properly
  • Corroded relay connector or earth
  • Broken control wire between the ECU and the relay
  • 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 P0685?

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

Check the battery terminals are clean and tight and look over the engine fuses. On many cars you can hear the main relay click in the under-bonnet fuse box when you switch the ignition on and off.

How to read and clear P0685 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 P0685 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

P0685 is a generic code, so it means the same on any make, for example on Citroën, Hyundai, Mercedes-Benz, Mini, Renault, Volvo. See what Final OBD does on your car in makes and models.

What does code P0685 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 control circuit for that relay is open: the command is not getting through or the relay is not responding.

Can you drive with P0685?

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 P0685?

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, P0685 comes back.