Diagnose and fix Honda P0685
Honda P0685 is a commonly misdiagnosed problem because the trouble code leads you to believe the problem is in the ECM. That’s because the code is defined as: P0685 PCM Power Control Circuit/Internal Circuit Malfunction. But first you have to understand how the ECM works.
Honda Job aid to help diagnose Honda P0685
The Honda Job Aid is for the following vehicles and is designed to help you understand Honda P0685 trouble code and provide diagnostic advice.
Vehicles affected by Honda Job Aid
2005–07 Accord Hybrid
2006–12 Civic and Civic Hybrid
Conditions for setting Honda P0685
• The ECM detects an internal Circuit malfunction.
• The ECM detects an improper ECM shut-down process. This is what causes the most confusion when diagnosing this code.
The ECM has a specific shut-down routine. When the ignition key is turned to the OFF position, the ECM stays on for a short period to perform some diagnostic tests. This takes a few milliseconds. Once complete, the ECM cuts the ground signal (MRLY) PGM-FI main relay control coil, which opens the contacts, turning the main relay off.
When you start the vehicle, the ECM sees battery voltage from IG1 and then provides ground (MRLY) to the PGM-F1 relay control coil, which activates PGM-FI main to provide power to the ECM on (IGP), to the injectors and to other relays.
If the IGP circuit loses voltage or that voltage intermittently drops too low while the ECM is commanding the main relay ON, the ECM shuts down, setting P0685.
1. Turn the ignition switch to ON (II).
2. Clear the trouble code
3. Start the engine, and let it idle for 30 seconds.
4. Turn the ignition switch to LOCK (0).
5. Turn the ignition switch to ON (II).
6. Check for pending or confirmed P0685.
7. Check the battery condition and replace the battery if needed.
8. Check the positive and negative battery cable terminal connections and the ground connections at the engine and the transmission.
9. Start the engine and let it idle. Then turn on the headlights on high beam, and turn on the A/C with the fan on high. This will warm up the battery cables and other electrical components to prepare them for hot (normal) condition testing. Let the engine idle for 30 minutes. Then turn it off.
10. Turn the ignition switch to ON (II), and leave the headlights on and the fan on high.
11. Do a voltage drop measurement of the positive and negative battery cables. Flex the cables while monitoring the voltage drop. If the drop is more than about 0.3 volts, replace the faulty cable.
12. Turn the ignition switch to LOCK (0) and turn the headlights off.
13. Do these checks:
• Check for an intermittent short to ground in the positive battery cable and the B+ alternator cable.
• Check the PGM-FI main relay 1 terminals in the under-hood fuse/relay box. If faulty, replace the under-hood fuse/relay box.
• Check the PGM-FI main fuses and their connections.
• Check the ground circuits between terminal G101 and the ECM/PCM. Make sure the proper thread-cutting bolt is in place at the ground terminal.
• Check for poor or loose connections (potential opens) in the following circuits between the ECM and PGM-FI main relay 1 in the under-hood fuse/relay box.
14. Test PGM-FI main relay 1, or substitute a known-good relay.
Two most common fixes for Honda P0685
Since the ECM must see good voltage on MRLY and IGP, shops are finding that a bad battery or bad alternator are the two most common causes of the low voltage signal.
Try performing all the steps above including swapping the PGM #1 relay. But also check for the battery condition and alternator.
©, 2022 Rick Muscoplat
Posted on by Rick Muscoplat