P0685 to P0690, P0634 and P0666 to P0669: engine computer power relay and internal temperature
The circuit that controls or monitors the engine computer's main power relay is open, low or high, or the computer's internal temperature is too high or its internal temperature sensor reads wrong.
SeverityHighCarrying on can cause expensive damage, so deal with it now.
Can you keep driving?Only as far as the workshopWhat does P0685 mean?
Many cars power the engine computer and its sensors through a main relay that the computer itself switches on and keeps on for a while after the engine stops, to save learned values and run fans. If the relay control or its feedback is wrong, the computer may lose power while driving, fail to start the engine or drain the battery by staying awake. The internal temperature codes protect the electronics: a computer mounted near the engine or exposed to sun and heat can overheat.
What causes P0685?
- A failed main relay, or corroded relay contacts
- A blown fuse in the relay circuit
- A poor ground or a corroded connector at the engine computer
- An engine computer mounted where heat builds up, or with its cooling blocked
- A fault inside the engine computer
What to check first for P0685
- Swap the main relay with an identical one and see whether the code returns
- Check the fuses for the relay and the engine computer
- Inspect the computer's grounds and connectors for corrosion
- Check whether the battery drains overnight, a sign that the relay stays on
Can you drive with P0685?
Drive only to a workshop. A main relay fault can stop the engine without warning, and every sensor goes with it; have the car towed if the engine has cut out or will not start.
In states with an OBD-II emissions inspection, a car with the check engine light on fails the test until the fault is fixed.
Codes on this page
11
- P0634PCM/ECM/TCM Internal Temperature Too High
- P0666PCM/ECM/TCM Internal Temperature Sensor Circuit
- P0667PCM/ECM/TCM Internal Temperature Sensor Range/Performance
- P0668PCM/ECM/TCM Internal Temperature Sensor Circuit Low
- P0669PCM/ECM/TCM Internal Temperature Sensor Circuit High
- P0685ECM/PCM Power Relay Control Circuit/Open
- P0686ECM/PCM Power Relay Control Circuit Low
- P0687ECM/PCM Power Relay Control Circuit High
- P0688ECM/PCM Power Relay Sense Circuit/Open
- P0689ECM/PCM Power Relay Sense Circuit Low
- P0690ECM/PCM Power Relay Sense Circuit High
Related codes
Checked on · Rafael Soares
These are the generic meanings every manufacturer shares. Each brand adds its own detail, so your car's handbook, its service information and a qualified mechanic have the last word. OdoKeep has no connection with any manufacturer, and the app does not read codes from the car.
