P0685: Engine Control Module Power Relay Control Circuit Open
What to do
May not start at all, or may cut out. The relay is inexpensive and a frequent cause, so ask for it to be tested first. If the vehicle has also been losing its learned settings or storing memory faults, that fits the same relay opening too early during shutdown.
What this code means
An open circuit in the control of the main engine module power relay. This relay is what supplies the module and, on most vehicles, the sensors and actuators around it — and the module holds it energised itself, releasing it only after the ignition is switched off and it has finished writing to memory.
That self-holding arrangement explains two symptoms that otherwise seem unrelated. A relay that opens too early cuts power mid-shutdown, which is one of the ways keep-alive memory becomes corrupted. A relay that does not energise at all produces an engine that cranks with no injection or spark, since everything downstream is unpowered.
The relay itself is cheap and a common failure, and it is worth testing before anything else in this group.
Likely causes
Start here
- Failed ECM power relay · easy
- Corroded relay socket · easy
- Broken wire in control circuit · moderate
- Corroded or loose connector · 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
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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.
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