P2639

Fuel Pump B Control Circuit High

P2639 is a generic OBD-II powertrain diagnostic trouble code: Fuel Pump B Control Circuit High. It is logged by the engine control unit when the powertrain monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.

Code
P2639
Group
Powertrain
System
Powertrain
Severity
Warning (MIL on)
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What P2639 means

P2639 is stored when the PCM detects a higher-than-expected voltage on the fuel pump B control circuit. This is the high-side counterpart to P2638 and typically indicates a short to a voltage source in the control wiring, a failed FPDM outputting excessive voltage, or an open in the ground side of the control circuit causing the signal to float high.

On PWM-regulated pump systems a high circuit condition can cause the control module to command full pump output continuously, potentially overpressurizing the fuel rail or running the pump beyond its designed duty cycle. On simpler relay-controlled systems the high voltage may prevent the relay from being switched off, locking the pump in the on state.

Diagnosis involves identifying whether the high voltage originates from the PCM output, a wiring fault, or the FPDM. A scan tool command test can help determine if the PCM can modulate the control signal; if the signal stays high regardless of commanded state, the fault is likely in the external circuit rather than the PCM.

Common causes

Most-frequently reported root causes when P2639 is logged.

  • 1
    Short to battery voltage on the pump B control wire.
  • 2
    Open ground circuit causing the control line to float high.
  • 3
    Failed FPDM outputting a continuous high-voltage signal.
  • 4
    Damaged wiring insulation contacting a voltage source.
  • 5
    Failed PCM driver stage locking the output high.
  • 6
    Incorrect wiring splice or repair introducing a voltage-source short.
  • 7
    Corroded control-circuit ground connection at the module or chassis.

Symptoms drivers notice

MIL illuminated.
Fuel pump B may run continuously regardless of ignition state.
Abnormally high fuel rail pressure possible if pump is unregulated.
Fuel system may not shut down correctly after key-off.
Possible battery drain if pump runs with ignition off.

How to diagnose P2639

A typical diagnostic flow when this code is present.

  1. 1
    Retrieve all DTCs and freeze-frame data; check for accompanying high-pressure or rail-pressure codes.
  2. 2
    With the circuit commanded off, backprobe the pump B control wire and measure voltage; a high reading confirms the short-to-power condition.
  3. 3
    Disconnect the FPDM and check whether the high voltage is present on the harness side or only at the module output.
  4. 4
    Inspect the control wiring harness for contact with battery-positive wires or fuse box terminals.
  5. 5
    Verify the ground integrity of the FPDM and its PCM command-signal ground.
  6. 6
    Perform a PCM output test via scan tool to confirm the driver can pull the signal low correctly.
  7. 7
    Replace FPDM, repair wiring short, or address PCM driver fault as test results indicate.

Related powertrain codes

Frequently asked questions

Is it dangerous to drive with P2639 if the pump runs constantly?

It can be. Continuous pump operation may overpressurize the fuel system or drain the battery; have the vehicle inspected promptly.

Can P2639 cause a rich fuel condition?

Yes. If continuous high pump output overwhelms the pressure regulator, excess fuel pressure can force injectors to deliver more fuel than commanded, causing a rich mixture.

How do I tell if the FPDM or PCM is at fault?

Disconnect the FPDM and check if the high voltage disappears from the harness; if it does the PCM and wiring are clear and the FPDM is faulty. If voltage remains on the harness side, trace for a short to power in the wiring.

Will the fuel pump burn out if P2639 is ignored?

Extended full-duty operation can accelerate pump motor wear and reduce its service life, especially if the pump was designed for intermittent operation cycles.

Disabling P2639 in software

RaceTune can permanently disable P2639 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.

Permanent
The monitor is disabled in the ECU itself — not just cleared. It cannot return.
Tailored to your file
Each patch is matched to your specific software version — never a one-size-fits-all file.
Reversible
The original file is always preserved. Reflash the stock to return the ECU to factory state.

ECUs with a P2639 disable in our catalogue

Confirmed coverage from our recipe database — we support many more families. Upload your file and our identifier will match it automatically.

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Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.

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