P0685 means ECM/PCM Power Relay Control Circuit/Open. The computer has detected a fault in the circuit that controls its own main power relay. It is a generic OBD-II code, so the definition is the same on any make, but the most likely cause varies by engine; confirm with a scan tool and the service information for your vehicle before buying parts.
What P0685 means
Modern vehicles have many control modules that share information over a network, usually called the CAN bus. Codes starting with U mean a module has stopped hearing another one, and voltage and processor codes mean a module is not getting clean power or has an internal fault. Low battery voltage, poor grounds, blown fuses and damaged wiring cause many of these codes, so test the battery and charging system first.
P0685 symptoms
- No start or stalling
- Check engine light
- Battery drain on some vehicles if the relay stays on
Common causes of P0685
- Failed main relay
- Wiring or fuse fault
- Corroded relay socket
How to diagnose P0685
- Check the main relay and its fuses
- Swap the relay with an identical one as a test
- Check the control circuit
Can you drive with P0685?
Not reliably; the engine may stall or not restart.
P0685 tips
The main relay powers the computer itself, so a failing relay can cause intermittent stalling that seems random. Swapping it with an identical relay in the same fuse box is a quick test.
Will P0685 clear itself?
If the fault stops, the computer turns off the check engine light by itself after several drive cycles without seeing it again, and the code stays in memory as history for a while. Clearing P0685 with a scan tool after a repair is fine, but if the cause is still there the code returns, sometimes within one drive. Disconnecting the battery also clears codes on most vehicles, but it erases learned fuel trims and idle settings as well.
Related codes
| Code | Meaning |
|---|---|
| P0562 | System Voltage Low |
| P0560 | System Voltage |
| P0606 | Control Module Processor |
Guides that help
When it is more than a sensor
Voltage and network codes come from wiring, the battery, the charging system or a module, not from engine or transmission wear. If a lost-communication code has left the transmission in limp mode, rule out the module and wiring before deciding the unit has failed, and see our transmissions only if testing confirms internal damage.
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