U0109: lost communication with the fuel pump control module

Older vehicles ran the fuel pump flat out through a relay. Modern ones vary pump speed with a dedicated control module, which is more efficient and keeps the fuel cooler.

That module sits on the network, and U0109 means it is not answering.

Related: P0627 to P0629 fuel pump control circuit.

Module problem or network problem?

The first question, and it changes the whole approach.

If only U0109 is present, the fuel pump module specifically is unreachable — its power, its ground, its connector or the module itself.

If several U-codes are present, the network or the vehicle’s power supply is the issue, and this module is one of several casualties: U0073 control module bus off and U0100 lost communication with ECM.

Count the codes before diagnosing anything.

Where the module lives, and why that matters

Fuel pump control modules are typically mounted under the vehicle: near the tank, above the rear axle, or in a rear wheel arch. Sometimes under the rear seat.

That location means:

• Road spray and salt on the connector.
• Water ingress in vehicles where the boot seal or rear light gaskets leak.
• Stone damage to the housing.
• Corrosion of the mounting, letting the module hang and stress the wiring.

Connector corrosion is the leading cause of this code on vehicles over about eight years old in salted-road climates.

Salt and corrosion protection and water in the spare wheel well.

Symptoms

• Cranks but does not start. The pump is not being commanded: engine cranks but won’t fire.
• Starts then stalls as residual rail pressure runs out.
• Power loss under load, if the module cannot command full output.
• Intermittent stalling, which is the version that resists diagnosis.
• Long cranking in the morning.

The listening test still works

Switch the ignition on without cranking, and listen at the rear of the car.

What you hear	Meaning
A short prime, one to three seconds	Module and pump both working
Nothing at all	Module not commanding, or no power
Continuous running	Module commanding constantly — control fault

If the pump primes normally and the code is stored, the fault is intermittent and the module is dropping off the network occasionally.

What to check

1. Read all codes and count how many modules are unreachable.
2. Check system voltage under load: P0560 to P0563 system voltage faults.
3. Locate the module — under the car, near the tank on most vehicles.
4. Inspect its connector for corrosion and water.
5. Check its power fuse and ground.
6. Check the network wiring to it, particularly where it passes the rear suspension.
7. Test the pump itself before replacing the module — a shorted pump can destroy the module’s output stage.

That last point matters. If a module has failed, ask why. Fitting a new one to a shorted pump destroys the new one too.

The data that helps

Fuel rail pressure during cranking.

Observation	Meaning
Pressure builds normally	Fuel delivery working — look elsewhere
No pressure at all	Pump not running
Pressure builds then falls	Pump running briefly, then stopping

P0087 fuel rail pressure too low.

Prevention, where it applies

Habitual low-tank driving shortens pump life, because the fuel around the pump provides cooling, and sediment at the bottom of the tank gets drawn in more often.

A pump that fails and shorts can take the control module with it — so a module failure sometimes traces back to years of running on the warning light.

P0313 and P0314 misfire with low fuel.

You can check this yourself. Pair an ELM327 adapter, open the free tier of the app and read every stored code before drawing conclusions. One unreachable module and five unreachable modules are entirely different problems.