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P0688: ECM/PCM power relay sense: circuit open

The ECU commands its main relay on but does not see the voltage coming back, due to a fuse, a worn relay or a broken wire.

Code P0688 at a glance
CodeP0688
SystemEngine and transmission
GroupP06xx, Control unit and auxiliary outputs
TypeGeneric: it means the same on every make
Can you keep driving?With care

What P0688 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. As well as switching the relay, the ECU checks that voltage is present at its output. This code means it gave the command but that voltage is not reaching it.

Most common causes

  • Blown ECU supply fuse
  • Low battery or loose terminals
  • Main relay with burnt or worn contacts
  • Corroded relay or ECU connector
  • Broken wire between the relay and the ECU
  • 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 P0688?

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 engine fuses one by one and the battery terminals. If the fault comes and goes over bumps, it points to a loose contact at the relay or its connector.

How to read and clear P0688 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 P0688 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

P0688 is a generic code, so it means the same on any make, for example on Abarth, Alfa Romeo, Cupra, Jeep, Nissan, Škoda. See what Final OBD does on your car in makes and models.

What does code P0688 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. As well as switching the relay, the ECU checks that voltage is present at its output. This code means it gave the command but that voltage is not reaching it.

Can you drive with P0688?

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 P0688?

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, P0688 comes back.