U0109

Lost Communication With Fuel Pump Control Module

U0109 is a generic OBD-II network diagnostic trouble code: Lost Communication With Fuel Pump Control Module. It is logged by the engine control unit when the network monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.

Code
U0109
Group
Network
System
Network
Severity
Critical (limp mode / no-start)
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What U0109 means

U0109 is set when the PCM or a network master detects that communication with the fuel pump control module (FPCM) has been lost. On vehicles equipped with a dedicated FPCM, this module manages fuel pump speed via pulse-width modulation to maintain target fuel pressure efficiently across varying engine load conditions. The PCM commands a fuel pressure target over the CAN bus, and the FPCM responds by modulating pump output accordingly while reporting actual fuel pressure and pump status back to the PCM.

Loss of FPCM communication means the PCM cannot regulate fuel pressure dynamically. Some FPCMs default to a fixed high-speed fuel pump operation on loss of communication, allowing limited engine operation; others default off, causing immediate fuel starvation. The outcome therefore depends on the specific fail-safe strategy implemented by the manufacturer, but in either case a no-start or stall condition is possible and driveability will be compromised.

Given the no-start implications, diagnosis should prioritize confirming whether the fuel pump is actually operating before extensive CAN bus diagnosis, then follow with standard network fault isolation: power, ground, bus integrity, and module response testing.

Common causes

Most-frequently reported root causes when U0109 is logged.

  • 1
    Open or short in the CAN bus wiring harness segment serving the FPCM.
  • 2
    Blown supply fuse or relay for the fuel pump control module.
  • 3
    Failed FPCM internal CAN transceiver or control processor.
  • 4
    Corroded or damaged FPCM connector, often located in the trunk or under-body near the fuel tank.
  • 5
    Heat or vibration damage to the FPCM mounting location.
  • 6
    Loose or high-resistance FPCM ground connection.
  • 7
    FPCM entering protection mode due to over-current from a failing fuel pump motor.

Symptoms drivers notice

Engine no-start or stall due to fuel pressure loss.
MIL illuminated; possible additional fuel pressure or pump codes.
Scan tool unable to communicate with the FPCM.
Fuel pressure may be absent, below specification, or at a fixed non-modulated level.
Possible extended crank time before start if FPCM defaults to a reduced output strategy.

How to diagnose U0109

A typical diagnostic flow when this code is present.

  1. 1
    Retrieve all DTCs; note whether fuel pressure DTCs accompany U0109.
  2. 2
    Verify with a fuel pressure gauge whether the pump is producing any pressure.
  3. 3
    Check the FPCM supply fuse, relay, and battery voltage and ground at the module connector.
  4. 4
    Inspect the FPCM connector and wiring for corrosion, damage, or moisture intrusion.
  5. 5
    Test CAN bus differential signal at the FPCM with an oscilloscope.
  6. 6
    Attempt a scan tool module scan to confirm the FPCM is absent from the network.
  7. 7
    Replace the FPCM only after confirming that bus, power, and ground integrity are all satisfactory.

Related network codes

Frequently asked questions

Will a car with U0109 always be a no-start?

Not always. Some FPCMs default to continuous full-speed operation on loss of communication, allowing the engine to start and run at the cost of fuel pressure regulation accuracy. Others default off, causing a no-start.

Can a failing fuel pump motor cause U0109?

A severely failing pump that overloads the FPCM's output stage can trigger the module's over-current protection and take the FPCM offline, producing U0109 alongside fuel pressure codes.

Is the FPCM the same as the inertia fuel shutoff switch?

No. The inertia shutoff switch is a passive safety device that cuts fuel power after a collision impact. The FPCM is an active electronic controller that modulates fuel pump speed in normal operation.

Where is the FPCM typically located?

The FPCM is commonly mounted near the fuel tank, either in the trunk area, under the rear seat, or under the vehicle. Its location makes it susceptible to moisture ingress and connector corrosion.

Disabling U0109 in software

RaceTune can permanently disable U0109 — 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.

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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