The code P0685 on a 2006 Honda Odyssey points to a fault in the ECM power relay control circuit. In practical terms, it can prevent the engine from starting or cause it to stall, and it typically involves the relay, wiring, or the ECM itself.
What P0685 means for the 2006 Honda Odyssey
In Honda vehicles, P0685 generally refers to the ECM power relay control circuit. The engine control module (ECM) relies on a stable power supply that is switched on by a relay; when the ECM cannot energize that relay or cannot receive a steady supply, this code is triggered. The issue could lie in the relay itself, the wiring, a blown fuse, or even the ECM that controls the relay.
Common causes
These are the most frequent culprits technicians check first when P0685 appears.
- Faulty ECM power relay or main relay
- Blown fuse or damaged fuse in the ECM power circuit
- Damaged or corroded wiring between the fuse box, relay, and ECM
- Loose, dirty, or misconnected connectors at the ECM or relay
- Weak or fluctuating battery voltage or poor grounding
- Internal fault within the ECM (less common, but possible)
Addressing these common causes promptly can restore reliable operation and prevent non-start conditions.
Typical symptoms
Owners may notice a range of symptoms when P0685 is present. Look out for these indicators.
- Engine cranks but does not start, or starts intermittently
- Engine stalls during operation or shortly after starting
- Vehicle may fail to start after sitting; the starter may crank but ECM does not energize
- Diagnostic trouble code P0685 stored in the PCM/ECM
- Electrical issues such as dash indicators or accessories behaving strangely due to power supply problems
If you observe these symptoms, have the vehicle checked by a technician to confirm the fault and avoid getting stranded.
Diagnosis and common repairs
To confirm P0685 and implement the fix, technicians follow a methodical process around the power relay circuit.
- Inspect fuses in the under-hood fuse/relay box related to the ECM power circuit and replace any blown fuses
- Test the ECM power relay coil and contacts; replace the relay if it fails
- Verify battery voltage and charging system; ensure proper ground connections
- Inspect wiring and connectors between the fuse box, relay, and the ECM for damage or corrosion; reseat or repair as needed
- Check the ECM power input circuit for proper voltage supply; diagnose upstream causes if voltage is absent or unstable
- If hardware tests pass, consider the ECM itself as a potential fault; replacement or reprogramming may be necessary in rare cases
Repair approaches vary by root cause, but most typically involve replacing a faulty relay, repairing wiring, or addressing a battery/ground issue.
Is it safe to drive with this code?
Because P0685 pertains to the ECM’s power supply, a vehicle with this code can be unreliable or unable to start. If the Odyssey starts and runs, treat the issue as urgent and seek repair promptly. If the car is no-start or stalls, avoid driving and arrange for towing to a repair shop.
What to tell your mechanic
Clear information helps speed up diagnosis. Share these details with the technician:
- When the code appeared and whether the problem is intermittent
- Whether the vehicle starts or cranks but won’t start
- Any related symptoms (dim lights, relay clicking, or poor electrical function)
- Recent work or damage near the under-hood area, battery, or relays
Providing specifics helps the technician check the ECM power relay circuit more efficiently.
Summary
P0685 on a 2006 Honda Odyssey signals a fault in the ECM power relay control circuit. Common causes include a failing relay, blown fuses, damaged wiring, or a faulty ECM. Diagnosis typically involves testing the relay, fuses, battery/ground, and the wiring between the fuse box and the ECM. Prompt diagnosis and repair can prevent roadside failure and costly downtime.