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P0687ECM/PCM Power Relay Control Circuit High

The control circuit for the engine computer's main power relay is reading higher voltage than expected, so the relay cannot be switched correctly.

Definition

The PCM switches its main power relay by pulling the coil control circuit low and continuously compares the measured circuit voltage with the commanded state. P0687 sets when the control circuit stays high — the PCM cannot pull it down, or the wire is shorted to battery voltage. If the relay never energizes the result is a crank-no-start or stalling; if it is held energized by a short, the PCM and its loads stay powered after key-off and drain the battery. A MIL, multiple companion codes, and intermittent electrical behavior in relay-fed systems are common.

Common causes

  • Relay control wire shorted to battery voltage
  • Open in the control circuit preventing the PCM from pulling it low
  • Failed main power relay or corroded socket terminals
  • Damaged fuse block wiring
  • Failed PCM low-side driver

Common misdiagnoses

  • PCM replacement before wiring is tested
  • Battery replacement for the parasitic-drain symptom
  • Ignition switch replacement
  • Alternator diagnosis for the high-voltage reading

Troubleshooting steps

  1. 1.Measure the control circuit in both commanded states

    With the key on, the circuit should toggle between near battery voltage (relay off) and near 0 V (relay on) as the PCM commands it.

  2. 2.Check for a short to power

    Disconnect the PCM connector; if the control wire still shows battery voltage, trace the harness for a short to a powered circuit.

  3. 3.Test circuit continuity

    Verify the control wire is continuous from the relay socket to the correct PCM pin with near-zero resistance.

  4. 4.Substitute the relay

    Swap in an identical known-good relay and inspect the socket terminals for heat damage.

  5. 5.Check for after-shutdown drain

    If the relay is being held on, measure parasitic draw after key-off — a latched main relay typically shows a draw of several hundred milliamps or more.

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