P0686: Engine Control Module Power Relay Control Circuit Low
What to do
Driveable, but if the battery goes flat after the car has stood for a few days, this is a likely explanation — the module may be staying powered after switch-off.
What this code means
The engine module power relay control circuit is reading low — a short to ground, or a relay held energised. A relay that never releases keeps the module and its sensors powered after the ignition is off, which flattens the battery over a few days and is a genuine cause of a car that will not start after standing.
Likely causes
Start here
- Failed ECM power relay · easy
- Signal wire shorted to ground · moderate
- Corroded relay socket · easy
Less common
- Failed control module · specialist
Symptoms
Components involved
Information, not instruction
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
Be the first to write this up
Nothing posted about this yet. Whatever you found is worth writing down — even a dead end saves someone the same afternoon.
What makes one worth reading:
- The car — make, model, year and engine. The same fault behaves differently on each.
- What you measured, not only what you replaced.
- What actually fixed it — or that nothing did, which is worth knowing too.
Sign in or create an account to add what you found about P0686.