ECM/PCM Power Relay Control Circuit Low
The P0686 code indicates that the powertrain control module (PCM) has detected a low voltage condition on the control circuit of the ECM/PCM power relay. This relay supplies battery voltage to the engine control module (ECM) and other critical components. A low voltage reading suggests a short to ground, open circuit, or faulty relay, which can prevent the ECM from powering up properly, leading to no-start or drivability issues.
1. Most Common Causes
- Short to ground in the ECM/PCM power relay control circuit wiring
- Loose, corroded, or damaged connections or terminals in the relay circuit
- Faulty ECM/PCM power relay (internal coil short or failed control circuit)
- Excessive resistance or open circuit in the wiring between the ECM/PCM and the relay
- Insufficient battery voltage or poor ground at the control module
- Internal ECM/PCM failure in the relay control circuit driver
2. Symptoms You Will Notice
- Engine cranks but does not start
- Check Engine Light illuminated
- No communication with scan tool (ECM unresponsive)
- Intermittent stalling or no-start conditions
3. Step-by-Step Diagnosis
Step 1: Verify battery voltage is at least 12.6V and check for any blown fuses related to the ECM/PCM relay.
Step 2: Locate the ECM/PCM power relay and test for battery voltage at the relay coil and switch terminals using a multimeter.
Step 3: Check for continuity and shorts in the relay control circuit wiring between the PCM and relay.
Step 4: If wiring and fuses are good, replace the relay and clear the code. If the code returns, test the PCM output signal.
4. Frequently Asked Questions
Is it safe to drive with this code?▼
No, it is not safe. The code indicates a potential power loss to the ECM, which can cause the engine to stall unexpectedly or fail to start, leaving you stranded.
How much does it cost to fix?▼
Typically $50-$150 for parts and labor. A relay replacement is inexpensive, but if wiring repairs or PCM replacement is needed, costs can exceed $500.
5. Commonly Affected Makes
This is a generic OBD2 code, potentially applicable to any manufacturer using this diagnostic standard.