P0687: power-relay control high with an unresolved state conflict

P0687 identifies a short-to-power condition in the powertrain relay's low-side control driver. The cited table consistently labels the condition high voltage, but contains conflicting descriptions of the command state used for detection.

What is supported and what remains uncertain

The malfunction-criteria column says the voltage is outside its permitted range while the driver is on. The strategy prose later says monitoring occurs with the output off. Those statements cannot both define one command-state test, so this article does not select one as the verified universal condition.

The unambiguous entries describe a controller-specific voltage decision corresponding to less than 0.5 ohm between output and controller power. They also specify run/crank voltage above 11 volts and eight failing samples out of ten at 250 milliseconds per sample.

Resolve the state before testing

The practical next step is to consult the exact controller's service procedure for the commanded-state requirement. A circuit voltage cannot be judged independently of that state. Preserve both the relay command and the voltage observation so the correct procedure can interpret them together.

This code addresses the relay-control driver, not the separate relay-feedback measurement. It does not prove that relay contacts are welded or identify a required replacement. Verification must use the resolved command-state procedure and demonstrate a passing enabled diagnostic. The source conflict is retained for review; the supported high-circuit meaning remains useful without inventing an electrical test.

Source and applicability

Documented monitors in gm-2025-03a, model year 2025. These are module/calibration examples; vehicle, engine and transmission applicability must be confirmed independently. GM 2025-03a, page 419.