P0685high severityPowertrain
ECM/PCM Power Relay Control Circuit /Open
on 2009 Audi A8 4.2L V8 · AWD
What this code means
The PCM has detected an open condition in the power relay control circuit that supplies power to the ECM/PCM. This can cause no-start or intermittent shutdown.
Symptoms
- Engine no-start
- Intermittent stalling
- Loss of PCM power
Common causes
- Faulty PCM power relay
- Open control circuit wiring
- Corroded relay socket or connector
- PCM internal fault
Diagnostic steps
- Check PCM power relay operation
- Inspect relay control circuit for open
- Test relay and socket connections
- Verify PCM power and ground
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