What this code means
P0685 may mean the PCM is seeing an unexpected condition in its power control circuit or an internal related circuit.
What the vehicle may do
- The vehicle may have an intermittent start or run concern.
- The code may set with little obvious driveability change.
- The concern can be intermittent.
Possible fault areas
- Possible PCM power control circuit concerns
- Possible main relay or main fuse concerns
- Possible loose terminals, poor connections, or ground concerns
- Possible battery cable, battery, PCM software, or PCM-related concerns
Diagnostic path
Set up the diagnosis
On this Accord, P0685 points to a PCM power control circuit or internal circuit malfunction. In plain terms, the PCM may not be seeing the power control behavior it expects. The vehicle may have an intermittent start or run concern, or it may simply set the code with little obvious driveability change. Broadly, this can involve the PCM power feed side, the main relay or fuse area, connector and terminal condition, PCM grounds, battery connections, the battery itself, PCM software, or the PCM. Before getting into the test sequence, record the captured freeze data and any on-board snapshot, and review the basic troubleshooting information. If the code has been intermittent or does not come right back after clearing, pay close attention to loose terminals at the IGP line connectors before condemning the PCM.
Duplicate the code first
Start by turning the ignition switch to ON (II), or selecting ON mode with the engine start/stop button. Clear the code with the HDS. Then start the engine and let it idle for 30 seconds. After that, turn the ignition switch to LOCK (0), or select OFF mode, then turn the ignition back to ON (II), or select ON mode again. Now check Pending or Confirmed DTCs with the HDS and see whether P0685 is indicated.
If P0685 returns during problem verification
If P0685 is indicated during that check, the failure has been duplicated. At that point, update the PCM if it does not have the latest software, or substitute a known-good PCM, then continue into the repair verification sequence.
If P0685 does not return during problem verification
If P0685 is not indicated, treat it as an intermittent failure. Check the PGM-FI main relay 1, the PGM-FI main fuse, terminal fit and connection quality between the PCM and the under-hood fuse/relay box, PCM ground terminals and connections, battery cable connections, and the battery.
Verify the repair
Keep verification separate from the earlier fault isolation. Start the engine and let it idle for 30 seconds. Turn the ignition switch to LOCK (0), or select OFF mode. Then turn the ignition switch back to ON (II), or select ON mode. Check Pending or Confirmed DTCs with the HDS again, and see whether P0685 is indicated.
Verification result
If P0685 is still indicated and the PCM was updated, substitute a known-good PCM and recheck. If the PCM had already been substituted, go back to the beginning of the diagnostic path and run the sequence again. If P0685 is not indicated, troubleshooting is complete. If a known-good PCM was substituted, replace the original PCM. If other Pending or Confirmed codes are present, check what they mean next.
Takeaway
The key on P0685 is to prove whether the fault comes back after the controlled clear, idle, key-off, and key-on sequence. If it duplicates, handle PCM software or known-good PCM substitution as directed. If it does not duplicate, stay focused on intermittent power, relay, fuse, connector, ground, battery cable, and battery issues before making a hard call. For more diagnostic training, visit stepdiagnostics.com.
Final check
P0685 is often approached by first trying to duplicate the fault, then separating a repeatable PCM/software path from an intermittent power, ground, relay, fuse, connector, or battery-side concern.
For more guided automotive diagnostics, visit STEP Diagnostics.
