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P027DFuel Pump "B" Module Control Circuit High

The control circuit for the secondary ("B") fuel pump driver module is stuck at a higher voltage than the PCM expects, typically from a short to battery voltage or an open circuit.

Definition

The PCM sets P027D when the voltage it reads on the "B" fuel pump module control line exceeds the calibrated maximum — commonly a short to voltage, an open circuit on a pulled-up line, or an internally failed module. Depending on the failure, the pump may run at full speed constantly (raising pressure and noise), run erratically, or not respond to commands at all, and the MIL will illuminate. Some systems shut the pump down as a fail-safe, causing stalling or a no-start, and sustained over-pressure can flood the engine rich or stress the pressure regulator.

Common causes

  • Control wire shorted to battery voltage (chafed against a powered circuit)
  • Open control circuit read as high on a pulled-up input
  • Failed fuel pump driver module driving the line high internally
  • Corroded connector pins creating high resistance/open on the control line
  • PCM driver fault (rare)

Common misdiagnoses

  • Fuel pump replaced when the control wiring is shorted to voltage
  • Fuel pressure regulator replaced for pressure symptoms caused by a full-speed pump
  • Fuel pump relay replaced although it is not part of the PWM control line
  • PCM replaced before wiring continuity is verified

Troubleshooting steps

  1. 1.Read the control-line voltage at the module

    Backprobe key-on: battery voltage present while the PCM commands a partial duty cycle indicates a short to voltage or an open with internal pull-up.

  2. 2.Check circuit continuity end to end

    With both ends disconnected, verify less than 1 Ω from PCM pin to module pin; an open here explains a high reading on a biased circuit.

  3. 3.Test for a short to voltage

    With the PCM disconnected and key on, any voltage on the control wire means it is contacting a powered circuit — trace the harness at chafe points.

  4. 4.Watch fuel pressure behavior

    Pressure pegged above specification with the pump audibly running full speed corroborates a control line stuck high or a failed module.

  5. 5.Substitute the driver module last

    If the harness has continuity, no short to voltage exists, and power/ground are good, replace the fuel pump driver module and confirm the PCM regains duty-cycle control.

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