P0687: ECM/PCM Power Relay Control Circuit High

P0687 is set when the ECM detects that the voltage on its own power relay control circuit is higher than expected. The ECM controls its main power relay -- the relay that powers the ECM itself and other critical engine management components -- through a dedicated control wire. The ECM monitors this wire and expects to see a specific voltage state depending on whether it has commanded the relay open or closed. A persistently high voltage reading when the relay should be off tells the ECM that either the relay is stuck closed, there is a short to voltage in the control wiring, or the ECM's internal relay driver circuit has failed.

Practically speaking, the most common driver of P0687 is wiring. A chafed control wire that has contacted a constant 12V supply elsewhere in the harness will hold the circuit high regardless of what the ECM commands. A faulty relay with welded contacts is the second most common cause -- the relay remains closed even after the ECM withdraws its ground signal. One distinguishing symptom of a stuck-closed relay is the ECM and associated components remaining powered for an abnormal time after the ignition is switched off, or a battery drain when parked.

Start diagnosis by checking the relay itself -- swap it with a known-good relay of the same type if one is available in the fuse box. If the fault clears, the relay was the issue. If the code persists, inspect the relay control circuit wiring between the relay socket and the ECM for any points where it may have contacted a voltage source. ECM failure as a root cause is uncommon but does occur, typically after water ingress or overheating events.