P0686
ECM/PCM Power Relay Control Circuit Open
The control circuit for the ECM main power relay is open. Without this relay, the controller cannot maintain its keep-alive supply.
- Check engine light: Yes
- Emissions related: Yes
- Repair difficulty: medium
Symptoms
- Check engine light on
- Possible limp mode or no-start
- Multiple follow-up faults across modules
- Reset/relearn may be needed after repair
Common causes
Ordered by how often each turns out to be the cause.
- Main relay defectiveCommon
- Relay control wire openPossible
- Pin pushed back out of connector bodyLess likely
Parts involved
- ECM
- Main Relay
- Relay Wiring
P0686 is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.