ECM/PCM Power Relay Sense Circuit Open
The P0688 code indicates that the Engine Control Module (ECM) or Powertrain Control Module (PCM) has detected an open circuit in the power relay sense circuit. This circuit monitors the voltage output from the ECM/PCM power relay to ensure the relay is functioning correctly. When the circuit is open, the ECM/PCM does not receive the expected voltage signal, which can lead to various drivability issues.
1. Most Common Causes
- Faulty main power relay (PCM power relay) stuck in the open position
- Blown PCM or main relay fuse
- Open or short-to-ground in the power relay sense circuit wiring
- Corroded electrical connections or terminals at the relay or PCM
- Internally damaged Powertrain Control Module (PCM)
2. Symptoms You Will Notice
- Engine may not start or stalls
- Check Engine Light illuminated
- No power to ECM/PCM (no communication with scan tool)
- Battery drain or electrical issues
3. Step-by-Step Diagnosis
Step 1: Use a scan tool to confirm the code and check for other related codes.
Step 2: Inspect the ECM/PCM power relay and its fuse; replace if blown or faulty.
Step 3: Check wiring and connectors for damage, corrosion, or looseness; repair as needed.
Step 4: Test the relay sense circuit voltage with a multimeter; compare to manufacturer specs.
Step 5: If all else fails, test the ECM/PCM for internal faults (requires professional diagnosis).
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, and the ECM/PCM may lose power, causing unpredictable behavior. Have it diagnosed immediately.
How much does it cost to fix?▼
Typically between $50 and $150 for parts and labor, but costs can vary if the ECM/PCM needs replacement.
5. Commonly Affected Makes
This is a generic OBD2 code, potentially applicable to any manufacturer using this diagnostic standard.