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P0629Fuel Pump "A" Control Circuit High

The PCM has detected abnormally high voltage on the fuel pump control circuit, often meaning the circuit is shorted to power or the pump runs when it should be off.

Definition

The PCM switches the fuel pump through a relay or driver module and expects the control circuit to toggle between defined on and off voltage states. P0629 sets when the circuit reads high when it should be low — commonly a control wire shorted to battery voltage, a relay with welded contacts, or a failed driver holding the circuit up. Consequences include a fuel pump that runs continuously with the key off (draining the battery and stressing the pump), inability of the PCM to shut off fuel delivery, or in the open-circuit variant a no-start; the MIL illuminates. A pump the PCM cannot switch off is also a safety concern in a collision fuel-cutoff strategy.

Common causes

  • Fuel pump control wire shorted to battery or ignition voltage
  • Fuel pump relay stuck closed (welded contacts)
  • Failed fuel pump driver module holding the output high
  • Corroded connector allowing voltage backfeed into the control circuit
  • PCM driver circuit failure

Common misdiagnoses

  • Replacing the fuel pump when the control circuit or relay is at fault
  • Replacing the battery for a parasitic drain actually caused by a pump running key-off
  • Replacing the PCM before checking for a simple stuck relay
  • Replacing the ignition switch chasing the always-on pump symptom

Troubleshooting steps

  1. 1.Check for a pump that never shuts off

    With the key off, listen at the filler neck or clamp a low-amps probe on the pump feed; any current key-off confirms the circuit is being held on.

  2. 2.Test the fuel pump relay

    Pull the relay and see whether the pump stops; verify contact operation on the bench and inspect the socket for heat damage.

  3. 3.Check the control circuit for shorts to voltage

    With the relay/driver disconnected and key on, the control wire should show its designed bias, not full battery voltage; trace any short to a chafed harness section or damaged connector.

  4. 4.Test the fuel pump driver module (where fitted)

    Verify the FPDM's command input from the PCM toggles correctly and its output follows; an output stuck high with a toggling input condemns the module.

  5. 5.Verify PCM control last

    If the wiring, relay, and driver module all test good and the PCM's control terminal stays high when commanded off, the PCM driver has failed.

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