ECM/PCM Power Relay Sense Circuit Open
Code P0688 indicates the PCM/ECM has detected an open circuit in the power relay sense circuit, which is responsible for monitoring voltage supplied to the engine control module. This prevents the PCM from properly verifying its own power supply and can cause starting issues and engine performance problems.
What You Might Notice
- Delayed or no engine start
- Electrical accessories inoperative or malfunctioning
- Engine drivability issues or rough running
- Check engine light illuminated
- Vehicle may stall unexpectedly
Most Common Causes
- 1
Defective PCM power relay
The power relay that supplies voltage to the PCM has failed internally or is stuck open, preventing proper sensing circuit operation.
- 2
Blown fuse or fusible link
The fuse or fusible link in the PCM power circuit has blown, interrupting power supply to the relay sense circuit.
- 3
Open or shorted wiring
Damaged, corroded, or disconnected wiring between the power relay and PCM sense circuit creates an open connection path.
- 4
Poor electrical connections
Loose connectors, corroded terminals, or damaged connector pins at the relay or PCM prevent proper signal transmission.
How to Diagnose It
- 1
Check Defective PCM power relay — The power relay that supplies voltage to the PCM has failed internally or is stuck open, preventing proper sensing circuit operation.
- 2
Check Blown fuse or fusible link — The fuse or fusible link in the PCM power circuit has blown, interrupting power supply to the relay sense circuit.
- 3
Check Open or shorted wiring — Damaged, corroded, or disconnected wiring between the power relay and PCM sense circuit creates an open connection path.
- 4
Check Poor electrical connections — Loose connectors, corroded terminals, or damaged connector pins at the relay or PCM prevent proper signal transmission.
How to Fix It
- 1
Test and replace PCM power relay
Use a DVOM to test relay continuity and voltage output. If relay fails testing, replace it with an OEM equivalent relay for your vehicle model.
- 2
Check and replace blown fuses
Locate the main fuse or fusible link in the PCM power circuit using the fuse box diagram. Replace any blown fuses with same-amperage ratings.
- 3
Inspect and repair wiring
Visually inspect the wiring harness between the relay and PCM for cuts, burns, corrosion, or pinches. Repair damaged sections or replace the entire harness if compromised.
- 4
Clean connectors and verify connections
Disconnect and thoroughly clean all connectors related to the PCM power relay circuit with electrical contact cleaner. Ensure all connections are tight and corrosion-free before reassembling.
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