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P codepowertrainMedium severity

P0690ECM/PCM Power Relay Sense Circuit High

The engine computer sees battery voltage on its power-relay sense circuit when the relay should be off, meaning the relay is stuck on or the circuit is shorted to power.

Definition

The ECM/PCM main power relay supplies switched battery voltage to the PCM and many engine actuators, and the PCM watches that feed through a dedicated sense wire. P0690 sets when the sense circuit stays at or near battery voltage while the relay is commanded off. Symptoms range from none at all to a dead battery overnight (the stuck relay keeps the bus powered after key-off), stalling, hard starting, and an illuminated MIL, and other circuits fed by the relay often set companion codes.

Common causes

  • ECM/PCM power relay with stuck or welded contacts
  • Sense-circuit wiring chafed and shorted to battery voltage
  • Corroded or heat-damaged relay socket in the underhood fuse/relay box
  • Aftermarket accessory wiring backfeeding the switched-power bus
  • Faulty PCM sense-circuit input

Common misdiagnoses

  • PCM replaced when a stuck power relay is the actual fault
  • Battery replaced for the parasitic drain the stuck relay causes
  • Ignition switch replaced without testing the relay circuit

Troubleshooting steps

  1. 1.Scan all modules for codes

    Companion codes on other relay-fed circuits (injectors, coils, solenoids) confirm the whole switched bus is involved rather than one sensor.

  2. 2.Test the power relay

    Swap in a known-good identical relay or bench-test it — the coil should measure roughly 50–120 ohms and the contacts must open cleanly when de-energized.

  3. 3.Measure the sense circuit at key-off

    Backprobe the sense wire; it should drop to near 0 volts within a few seconds of key-off — sustained battery voltage means a stuck feed or short to power.

  4. 4.Inspect the relay socket and fuse box

    Look for melted plastic, discolored terminals, or spread terminal cavities that can hold the contact circuit closed or create intermittent feeds.

  5. 5.Check for parasitic draw

    After key-off and module sleep, total draw should typically fall below about 50 mA; a stuck ECM power relay will show a much larger continuous drain.

  6. 6.Verify PCM powers and grounds before condemning the PCM

    Confirm all PCM ground straps and power feeds show less than about 0.1 V drop under load before replacing the module.

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