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 main relay to the ECM/PCM. When the relay is commanded on, the ECM expects to see battery voltage on the sense circuit. If the voltage is missing or too low, the code sets. Common causes include a faulty relay, blown fuse, wiring issues, or a failing ECM/PCM.
01. Most Common Causes
- Faulty ECM/PCM power relay
- Blown fuse or fusible link
- Open or shorted wiring in the relay sense circuit
- Corroded or loose relay socket connections
- Failing ECM/PCM
02. Symptoms You Will Notice
- Engine cranks but does not start
- Check Engine Light illuminated
- Engine stalling or no power
- Battery drain or electrical issues
03. Step-by-Step Diagnosis
Step 1: Scan for codes and verify P0688 is present.
Step 2: Inspect the ECM/PCM power relay and its fuse; replace if blown.
Step 3: Check for battery voltage at the relay sense terminal with the relay commanded on.
Step 4: Test continuity and resistance in the sense circuit wiring from relay to ECM.
Step 5: If all else is good, test the ECM/PCM for internal faults.
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, 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 or ECM repair is needed, costs can be higher.