ECM/PCM Power Relay Control Circuit High
The P0687 code indicates that the Powertrain Control Module (PCM) has detected a high voltage condition in 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 voltage, a faulty relay, or a problem within the PCM itself. This can lead to intermittent or complete loss of engine control, causing stalling, no-start, or drivability issues.
01. Most Common Causes
- Faulty ECM/PCM power relay
- Short to voltage in the relay control circuit
- Corroded or damaged relay socket or wiring
- Failed PCM (internal short)
02. Symptoms You Will Notice
- Engine cranks but does not start
- Intermittent stalling or no-start conditions
- Check Engine Light illuminated
- Loss of communication with scan tool
03. Step-by-Step Diagnosis
Step 1: Use a scan tool to retrieve all codes and freeze frame data.
Step 2: Inspect the ECM/PCM power relay and its socket for corrosion, damage, or loose connections.
Step 3: Test the relay control circuit for voltage with a multimeter; compare to manufacturer specifications.
Step 4: Check for shorts to voltage in the wiring between the relay and PCM.
Step 5: If wiring and relay are good, test the PCM for internal faults or replace it as a last resort.
04. Commonly Affected Makes
05. Frequently Asked Questions
Is it safe to drive with this code?▼
No, it is not safe. The vehicle may stall or fail to start unexpectedly, leading to a potential accident or being stranded.
How much does it cost to fix?▼
Typically $50-$150 for relay and wiring repairs, but PCM replacement can cost $500-$1000 or more.