ECM/PCM Power Relay Control Circuit High
The P0687 code indicates that the Powertrain Control Module (PCM) has detected a high voltage condition on the control circuit of the ECM/PCM power relay. This relay supplies battery voltage to the PCM and other critical components. A high voltage reading suggests a short to power, a faulty relay, or an internal PCM issue. This can prevent the vehicle from starting or cause erratic electrical behavior.
1. Most Common Causes
- Short to battery voltage (B+) in the ECM/PCM power relay control circuit wiring
- Faulty ECM/PCM power relay (welded contacts or shorted coil)
- Corroded or damaged connections in the relay circuit causing abnormal resistance
- Internal ECM/PCM failure in the relay control circuit
- Incorrect or higher amperage fuse installed in the relay circuit
2. Symptoms You Will Notice
- Engine cranks but does not start
- No power to PCM (no check engine light, no communication)
- Intermittent stalling or no-start conditions
- Battery drain or electrical issues
3. Step-by-Step Diagnosis
Step 1: Check for blown fuses related to the PCM power relay circuit.
Step 2: Inspect the relay and its socket for corrosion, damage, or loose connections.
Step 3: Test the relay control circuit for voltage with a multimeter; should have battery voltage with key on.
Step 4: Swap the relay with a known good one (if same type) and see if code clears.
Step 5: Check for shorts to power in the wiring harness between the relay and PCM.
Step 6: If all else fails, test the PCM for internal faults (requires professional scan tool).
4. Frequently Asked Questions
Is it safe to drive with this code?▼
No, it is not safe. The vehicle may stall or fail to start, leaving you stranded. Have it diagnosed and repaired promptly.
How much does it cost to fix?▼
Typically $50-$150 for relay and fuse replacement. If wiring repair is needed, costs can increase to $200-$400. PCM replacement is rare and expensive ($500-$1000+).
5. Commonly Affected Makes
This is a generic OBD2 code, potentially applicable to any manufacturer using this diagnostic standard.