P2638

Fuel Pump B Control Circuit Low

P2638 is a generic OBD-II powertrain diagnostic trouble code: Fuel Pump B Control Circuit Low. 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
P2638
Group
Powertrain
System
Powertrain
Severity
Warning (MIL on)
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What P2638 means

P2638 is set when the PCM detects a lower-than-expected voltage or current on the fuel pump B control circuit. This code is a subcategory of P2637 and specifically indicates the circuit has measured below the minimum calibrated threshold, most commonly pointing to a short to ground, a high-resistance open, or a failed PCM driver pulling the signal low.

On systems where pump B speed is regulated by a PWM signal, a shorted control line can cause the driver module to see a permanently low duty-cycle condition, reducing pump output. On relay-controlled systems a low signal may indicate the relay coil circuit is shorted to ground, holding the relay open and preventing the pump from receiving power.

Diagnosis follows standard low-circuit electrical testing: measure voltage at the control wire with the circuit commanded on, check for shorts to ground, and verify the PCM output. Because this code can result from a faulty FPDM or relay as well as wiring, all components in the control path should be evaluated before condemning the PCM.

Common causes

Most-frequently reported root causes when P2638 is logged.

  • 1
    Short to ground on the pump B PWM control wire or relay coil circuit.
  • 2
    Failed PCM driver for the pump B output pulling the circuit low.
  • 3
    Internal failure of the fuel pump driver module creating a low-voltage output.
  • 4
    Corroded connector pins increasing resistance and dropping signal voltage below threshold.
  • 5
    Damaged wiring insulation allowing contact with chassis ground.
  • 6
    Faulty pump B relay with a shorted coil.
  • 7
    Water ingress into the PCM or FPDM connector.

Symptoms drivers notice

MIL illuminated.
Fuel pump B may not operate or operates at reduced speed.
Engine hard to start or stalls under load.
Low fuel rail pressure readings on scan tool live data.
Possible no-start if pump B is the sole active supply pump.

How to diagnose P2638

A typical diagnostic flow when this code is present.

  1. 1
    Connect a scan tool and document all codes and freeze-frame data.
  2. 2
    Check the pump B fuse and relay for continuity and correct operation.
  3. 3
    With the circuit commanded on, backprobe the control wire and measure voltage; a reading near zero indicates a short to ground.
  4. 4
    Disconnect the FPDM or relay and check whether the short clears to isolate the fault to the wiring or the module.
  5. 5
    Inspect the harness between the PCM and FPDM/relay for chafing or pinched sections.
  6. 6
    Measure PCM connector pin voltage with the harness side disconnected to test the internal driver.
  7. 7
    Repair wiring shorts or replace the FPDM/relay as directed by test results.

Related powertrain codes

Frequently asked questions

How does P2638 differ from P2637?

P2637 is a general control-circuit fault; P2638 specifically identifies the fault as a low-voltage or low-current condition in that circuit.

Can moisture cause P2638?

Yes. Water in the PCM connector or FPDM connector can create a low-resistance path to ground, pulling the control signal below the threshold and setting this code.

Is P2638 always a wiring problem?

Not always; a failed PCM output driver or a faulty FPDM can also produce a low-circuit reading without any wiring damage present.

What happens to the engine if pump B never runs?

Depending on the fuel system architecture, the engine may run on pump A alone with reduced performance, or it may not start at all if pump B provides necessary lift pressure.

Disabling P2638 in software

RaceTune can permanently disable P2638 — 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 P2638 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.

  • Bosch MD1CS001 verified 1 software version

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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