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P0628Fuel Pump "A" Control Circuit Low

The PCM has detected abnormally low voltage on the fuel pump control circuit, meaning the pump may not be receiving its commanded power.

Definition

The PCM controls the fuel pump through a relay or a dedicated fuel pump driver/control module and monitors the control circuit's voltage for feedback. P0628 sets when that circuit reads below the expected range while the pump is commanded — typically a short to ground, an open feed, excessive circuit resistance, or a failing pump dragging the voltage down. Symptoms include long cranking, low fuel pressure, loss of power under load, stalling, or a crank/no-start, and the MIL illuminates. Because the pump can cut out without warning, this fault can leave the vehicle stranded and should be diagnosed promptly.

Common causes

  • Corroded or high-resistance connections in the fuel pump circuit (relay socket, inline connectors, tank connector)
  • Fuel pump control circuit wiring shorted to ground or open
  • Failing fuel pump relay or fuel pump driver module (FPDM/FPCM)
  • Worn fuel pump motor drawing excessive current and pulling the circuit down
  • Blown fuel pump fuse or open feed circuit
  • PCM low-side driver fault (least common)

Common misdiagnoses

  • Replacing the fuel pump when a $20 relay or a corroded connector is the fault
  • Replacing the fuel filter for a low-pressure complaint that is electrical
  • Replacing the PCM instead of the separate fuel pump driver module on vehicles that have one
  • Replacing injectors or ignition parts chasing lean/stall symptoms caused by pump power loss

Troubleshooting steps

  1. 1.Verify the symptom and fuel pressure

    Listen for the 2-second pump prime at key-on and measure fuel pressure against spec; no prime or low pressure aligns with a control circuit fault.

  2. 2.Test the relay and its socket

    Swap in a known-good relay or bench-test the original, and check the relay socket terminals for heat discoloration and looseness — a very common failure point.

  3. 3.Measure voltage drop under load

    With the pump commanded on, measure voltage at the pump connector; more than about 0.5 V below battery voltage indicates resistance in the feed or ground to locate and repair.

  4. 4.Check the control circuit to the PCM/driver module

    Test the control wire for shorts to ground and continuity, and on Ford-style systems check FPDM power, ground, and duty-cycle command.

  5. 5.Measure pump current draw

    Use a low-amps clamp on the pump feed — an erratic waveform or current well above spec (typically 4-10 A depending on system) indicates a worn pump motor.

  6. 6.Repair the confirmed component

    Replace only the proven failure — wiring repair, relay, driver module, or pump — then confirm pressure and re-run the drive cycle.

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