The P0685 code on a 2007 Honda Odyssey signals a problem with the PCM power relay control circuit—typically a faulty main relay, a blown fuse, wiring issues, or a failing PCM.
In Honda vehicles of this era, the engine control module (ECM/PCM) is powered through a dedicated relay and feed circuit. When the relay doesn’t switch properly, the PCM may not receive power, causing no-start conditions, erratic operation, or communication failures with a scan tool. This article explains what P0685 means, common causes, symptoms, how technicians diagnose it, and what repairs are commonly required.
What P0685 means for the 2007 Odyssey
P0685 indicates an issue with the PCM power relay control circuit. In practical terms, the PCM (or ECM) is not being properly powered due to a fault in the relay that supplies it, a fuse in the circuit, or a problem in the wiring or the PCM itself. Because the PCM needs power to function, problems in this circuit can lead to no-start conditions, intermittent starting, or ECU communication errors.
Causes
Several electrical and control issues can trigger P0685. Understanding the most common culprits helps focus the diagnostic process:
- Faulty PCM power/main relay or a relay coil that will not energize
- Blown fuse or poor connection in the PCM power circuit
- Wiring damage, corrosion, or loose connectors in the PCM power relay circuit
- Poor grounding or a high-resistance ground affecting the PCM supply
- Internal PCM fault (less common, but possible)
Problems in any part of the power feed to the PCM can produce the P0685 code, so a systematic check of power, ground, and relay operation is essential.
Symptoms
Not all drivers will experience every symptom, but the following are commonly reported when P0685 is present:
- Engine does not start or has a long crank time
- Check Engine light and possibly immobilizer/security indicators
- Scan tool cannot communicate with the PCM or shows partial data
- Dash gauges and accessories may behave abnormally when the ignition is on
- Intermittent starting or stalling related to PCM power loss
If you notice these symptoms together with a P0685 code, focus on the PCM power feed and relay system as the likely root cause.
Diagnosis and fixes
Technicians approach P0685 by verifying power, ground, and relay operation, then narrowing down to wiring or PCM problems. The steps below outline a typical process:
- Inspect and test fuses and fusible links in the PCM power circuit, replacing any that are blown or corroded.
- Test the PCM power relay (main relay) for proper operation; replace if the relay or its coil is weak or stuck.
- Check the relays and connectors for corrosion, looseness, or damage; reseat or replace as needed.
- Verify battery health and charging system to ensure adequate voltage supply to the PCM; correct any low-voltage issues.
- Test the PCM power feed and ground pins with a multimeter to confirm proper voltage and a solid ground reference.
- Inspect wiring harnesses and connectors between the under-hood fuse/relay box and the PCM for damage or water intrusion.
- Using a scan tool, confirm there are no additional codes that could point to an intermittent power issue or immobilizer conflict.
- If all power and wiring checks pass but P0685 persists, evaluate the PCM itself for faults and consider professional replacement/reprogramming, noting that dealer reprogramming may be required.
Repair outcomes vary by vehicle condition, but common resolutions begin with replacing the main relay or a blown fuse, followed by wiring repairs. A stubborn issue may require PCM replacement, which often involves dealer reprogramming or immobilizer pairing to restore proper operation.
What to do next
If you see P0685 on your 2007 Honda Odyssey, start with the low-cost checks (fuses and relay) and then move to wiring and power/ground tests. Because the PCM is central to engine management, a professional inspection is advisable if you are not equipped to safely perform electrical diagnostics. A proper diagnosis will prevent unnecessary part replacements and ensure the correct component is repaired or replaced.
Summary
P0685 on a 2007 Honda Odyssey flags a problem in the PCM power relay control circuit. The issue is usually tied to the main relay, a fuse, or wiring feeding the PCM, but can occasionally involve the PCM itself. Diagnosis starts with fuses and relays, then power/ground checks, followed by wiring inspection and, if needed, PCM testing or replacement with appropriate reprogramming. Addressing these steps promptly can restore starting reliability and engine management functionality.