P0627 to P0629: fuel pump control circuit faults

Running a fuel pump flat out whenever the engine is on wastes energy and heats the fuel. Modern vehicles instead vary pump speed to deliver only the pressure the engine currently needs.

That requires a fuel pump control module, and it introduces failure modes that a simple relay never had.

Code	Meaning
P0627	Fuel pump control circuit / open
P0628	Circuit low
P0629	Circuit high

The relay-based equivalent: P0230 to P0232 fuel pump primary circuit.

Establish which system you have

The whole diagnosis depends on it.

Relay-based. A relay in the fuse box, a pump that runs at one speed, and a two-second prime you can hear. The relay-swap test applies.

Module-based. A fuel pump driver module, usually mounted under the car or near the tank, communicating with the ECU. Pump speed varies. There is no relay to swap.

If your vehicle sets P0627 to P0629, it almost certainly has a module.

Where the module lives, and why that matters

Fuel pump control modules are commonly mounted under the vehicle — near the tank, above the rear axle, or in a rear wheel arch.

That location means road spray, salt and grit, which is why corrosion of the module connector is one of the leading causes of these codes.

Check the connector before anything else. On a vehicle over eight years old in a salted-road climate, this is the first suspect.

Salt and corrosion protection and underbody rust inspection.

The network angle

Because it is a module, it communicates. A separate code covers losing that communication entirely:

U0109 — lost communication with the fuel pump control module: U0109.

If you see U0109 rather than P0627, the module may be fine and simply unreachable — a network wiring fault, a power supply problem, or a bus issue affecting several modules at once.

Check whether other modules are also unreachable. One quiet module is a module problem; five quiet modules is a bus or supply problem: U0073 control module bus off.

Symptoms

• Cranks but does not start, because pressure never builds: engine cranks but won’t fire.
• Starts and stalls as residual pressure runs out.
• Power loss under load if the pump cannot be commanded to full output.
• Long cranking in the morning.
• Intermittent stalling, which is the version that defies diagnosis until you catch it in a log.

The data that identifies it

Fuel rail pressure, commanded and actual, is the decisive pair.

Pattern	Meaning
Both track together	Fuel delivery healthy — look elsewhere
Commanded rises, actual does not	Pump not being driven, or pump failed
Actual falls only under load	Supply restriction or a pump losing capacity
No pressure at cranking	Nothing is driving the pump at all

Where the vehicle exposes it, fuel pump duty cycle is even more direct: duty pegged at 100% with low pressure means the module is trying and the pump is not delivering.

P0087 fuel rail pressure too low.

Rule out the cheap things first

Before condemning a module:

1. The fuse feeding it, and the fuse feeding the pump.
2. The connector, for corrosion and water.
3. Ground, which is often a body stud in a corroded area.
4. The pump itself, which can short and take the module’s output stage with it.

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

Fuel quality and pump life

Pumps run cooler and last longer when submerged in fuel. Habitual low-tank driving shortens pump life and draws in the sediment that settles at the bottom of the tank.

P0313 and P0314 misfire with low fuel and bad fuel quality OBD symptoms.

In what order

1. Confirm you have a module, not a relay.
2. Read all codes — U0109 changes the diagnosis.
3. Inspect the module connector for corrosion.
4. Check fuses and grounds.
5. Log rail pressure commanded against actual during cranking and under load.
6. Test the pump before replacing the module.
7. Replace the module last, after establishing the pump is not shorted.

You can check this yourself. Pair an ELM327 adapter, open the free tier of the app and log fuel rail pressure during a failed start. Whether pressure builds at all separates a pump problem from a control problem immediately.