HomeTrouble CodesP0688

P0688 — Ecmpcm Power Relay Sense Circuitopen

Generic OBD-II · all makesPowertrain (engine & transmission)

This diagnostic trouble code indicates the engine control module has detected an open circuit in the power relay sense line. It means the computer is not receiving the expected voltage signal confirming the main power relay is active. This condition often cuts power to critical engine management systems.

Symptoms

What causes it — most likely first

1. Blown Fuse

The fuse supplying power to the relay control or output circuit has failed.

2. Failed Power Relay

The main relay responsible for energizing the PCM has stopped functioning internally.

3. Damaged Wiring

The harness between the relay box and the computer has broken or chafed.

4. Corroded Connectors

Poor electrical contact exists at the relay socket or module plug.

5. Faulty Ground

A bad ground connection prevents the relay control circuit from completing.

6. Failed PCM

The internal circuitry of the control module cannot process the signal (rare).

How it's diagnosed

  1. Check all fuses related to the engine control system for continuity.
  2. Swap the ECM power relay with a known good unit of the same type.
  3. Inspect wiring harnesses for visible damage or corrosion near connectors.
  4. Measure voltage at the PCM connector pins with the ignition switched on.
  5. Test ground points associated with the powertrain control system.
  6. Monitor live data to verify relay command status versus actual voltage.

Code facts & related codes

SystemPowertrain (engine & transmission)
StandardizationSAE generic — identical on every make

Frequently asked

Can I drive the vehicle with this code?

No, the engine may not start or could stall without warning.

Do I need to replace the computer for this code?

Rarely. This issue is usually caused by fuses, relays, or wiring before the computer is at fault.

What causes the relay circuit to open?

Heat, vibration, and corrosion often degrade connections over time.

Will a low battery trigger this code?

Yes, insufficient voltage can prevent the relay from engaging properly.