OBD-II trouble code
P0628: Fuel Pump "A" Control Circuit Low
A fuel pump is the hungriest small motor on the vehicle, and a worn one gets hungrier as it dies. That is why this reading is so often a report on the pump's condition rather than on the wiring — and why the fuel level you habitually drive at is a genuine cause.
Quick facts
- System
- Powertrain
- Category
- Fuel & Air Metering
- Severity
- High severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $90 – $1,200
- DIY difficulty
- Intermediate DIY
What does P0628 mean?
P0628 is the module reporting that the fuel pump 'A' control circuit is reading low — the voltage on the line it uses to command the pump sits below what it expects for the state it commanded. Its neighbours cover an open or general fault in the same circuit (P0627) and a high reading (P0629).
The useful thing to know about this particular reading is how much of the time it is a report on the pump rather than on the wiring, and the reason comes down to current. A fuel pump is by some margin the hungriest small motor on a typical vehicle: it runs continuously whenever the engine is running, it is a brushed DC motor immersed in fuel, and a healthy one draws somewhere in the region of eight to twelve amps depending on the vehicle and the pressure it is holding. That is a lot of current to push through automotive-gauge wire, crimped terminals and a bolted ground eyelet.
When a pump wears — brushes shortening, commutator pitting, the armature starting to drag in bearings that have run for a decade and a couple of hundred thousand miles — it does not simply get weaker. It gets hungrier. The motor pulls more current to do the same work, and that extra current does two things at once: it heats every connection in the circuit, and it drags the supply down under load. A module watching this circuit sees the result as a reading that has fallen out of range. So a P0628 accompanied by a pump you can hear whining from outside the car, a fuel pump connector that is warm to the touch, or a vehicle that has become harder to start when the tank is low is very likely telling you about the pump, not about a wire.
That leads to the most important measurement on this code, which is not resistance. Testing the circuit for continuity with the key off will happily pass a connector that loses a full volt once it is carrying ten amps. The test that finds the fault is a voltage drop measurement taken while the pump is actually running: battery positive to pump supply on one side, pump ground to battery negative on the other. More than about half a volt on either side is a real finding and points straight at the connection that is doing it.
There is one physical detail worth knowing before you go looking, because it saves people a great deal of time. The highest-yield inspection point on this whole circuit is the connector at the top of the fuel pump module, and on most vehicles it is reachable through an access hatch under the rear seat base or the boot floor rather than by dropping the tank. It is also the connection that carries the pump's full current through terminals that live in a warm, sealed environment, so it is the one that browns, deforms and eventually melts. Burned pigtails at this connector are common enough that repair harnesses are manufactured specifically for them. That is a repair costing a fraction of a pump, and a great many people replace a perfectly serviceable pump because the melted connector on top of it was mistaken for evidence that the pump killed itself.
There is a cause here that is behavioural rather than mechanical and it is worth raising because it is free to change. The fuel in the tank is what cools an in-tank pump — the fuel flows around and through the motor on its way to the engine. A vehicle habitually driven with the needle near the bottom runs its pump hot for years, and heat is what kills brushes and bearings. This is a genuine contributor to premature pump failure, and it is the single most useful thing to tell somebody who is about to fit a new pump, because a new pump in an old habit fails the same way.
One other presentation deserves mention because it looks like a different problem entirely. On circuits where the module switches the pump on the ground side, a line held low keeps the pump commanded whenever it has supply — so the pump can run with the key off. That shows as a flat battery in the morning, a faint humming from the rear of a parked car, or unusually warm fuel. If that is what you have, pulling the relay or disconnecting the driver and seeing whether the pump stops separates a stuck relay or a shorted control wire from a module output that is not releasing.
The vehicle will usually still drive with this stored, which is why it is not marked undriveable, but it should not be treated as something to live with. A pump drawing excess current is a pump approaching the end, and the way that ends is a stall in traffic rather than a gradual decline.
Common causes
- Worn fuel pump drawing excess current as brushes and bearings wear
- Burned or melted connector at the top of the fuel pump module
- High-resistance ground for the fuel pump circuit at the rear of the vehicle
- Fuel pump control wire shorted to ground against the underbody or a bracket
- Corroded terminals in the fuel pump relay socket
- Habitual low-fuel operation causing chronic pump overheating
- Clogged fuel filter or blocked in-tank strainer making the pump work harder
- Stuck fuel pump relay holding the pump commanded
- Failed fuel pump control module with a shorted output stage
- Chafed underbody harness bridging the control line to chassis
Symptoms
- Audible whine or drone from the fuel pump area, especially when the tank is low
- Extended cranking before the engine starts
- Hesitation or a loss of power under load, on hills or when accelerating hard
- Engine stalling at low fuel levels and restarting after refuelling
- Fuel pump connector or fuse warm or discoloured
- Battery flat overnight with a faint hum from the rear of the vehicle
- Blown fuel pump fuse recurring after replacement
- Check engine light stored alongside lean or fuel pressure codes
Diagnostic steps
- 1.Test voltage drop along the fuel pump circuit with the pump actually running, not resistance with the key off. This is the measurement that matters and it is the one most often skipped. A circuit can pass a continuity check and still lose a full volt across a degraded connector once it is carrying the eight to twelve amps a working pump draws. Measure between the battery positive post and the pump's supply terminal, and separately between the pump's ground terminal and the battery negative post, with the pump under load. Anything much over half a volt on either side is a finding.
- 2.Measure the pump's current draw. A healthy in-tank pump has a fairly predictable appetite, and a pump with worn brushes and a dragging armature pulls noticeably more. Excess current heats every connection in the circuit, drags the supply down, and is exactly what produces an out-of-range reading on the control line. High current with a healthy-looking circuit is the pump telling you it is on the way out, and no amount of wiring repair will change that.
- 3.Get to the fuel pump module connector at the top of the tank and look at it closely. On most vehicles this is reachable through an access hatch under the rear seat base or the boot floor rather than by dropping the tank. Look for browning around the terminals, melted or deformed plastic, and green or white corrosion. A burned pigtail here is common enough that dedicated repair harnesses are sold for it — and it is a repair, not a reason to buy a pump.
- 4.Ask how the vehicle is normally driven, specifically how low the tank is usually allowed to get. The fuel in the tank is what cools an in-tank pump, and a pump that spends its life sitting in two inches of fuel runs hot for years. This is a real cause with a free remedy, and it is worth mentioning to anyone about to fit a new pump, because the new one will die the same way if nothing changes.
- 5.Disconnect the pump and read the control line with the load removed. If the line still reads low with nothing connected, the fault is upstream in the harness, the relay or driver, or the module. If it returns to a normal resting value, the load was pulling it down and the pump or its ground is what you are looking for. This is a quick split and it decides which half of the circuit gets the rest of your time.
- 6.Check whether the pump is being commanded when it should not be. A control line held low can leave the pump energised whenever it has supply, which shows as a pump you can hear running with the key off, a battery flat overnight, or fuel that is unusually warm in the tank. If that is the presentation, remove the relay or disconnect the driver and see whether the pump stops — that separates a stuck relay or a shorted control wire from a module output that is not releasing.
- 7.Inspect the fuel pump ground point at the rear of the vehicle. It is usually a bolted eyelet on the underbody or in the boot well, it lives in the wettest and saltiest part of the car, and a ground that has gone high-resistance changes every reading on the circuit. Clean to bright metal and use a star washer rather than simply retightening a corroded stud.
Repair cost
$90 – $1,200
Diagnosis is $90 to $200, and should include a voltage drop test with the pump running rather than a resistance check — the difference decides whether you buy a pump or a connector. Repairing or replacing a burned fuel pump connector pigtail is $90 to $280 and is a very common and much cheaper outcome than the pump itself. Cleaning and re-terminating a corroded ground is $60 to $150. A fuel pump relay is $15 to $90. A complete in-tank fuel pump module is $350 to $1,100 fitted, at the lower end where the vehicle has an access hatch under the rear seat and at the upper end where the tank has to be dropped. A fuel pump control module, where fitted, is $120 to $600. Replacing the fuel filter at the same time as a pump is usual and adds $30 to $150.
Estimate your repair
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Open the Repair Cost Estimator with fuel pump replacement preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.