P0628 Code: Fuel Pump "A" Control Circuit Low
The fuel pump control wire is pulled toward ground – a chafed wire, shorted relay coil or wet fuse box.
At a glance
Severity: Moderate
- Meaning
- Fuel Pump "A" Control Circuit Low
- System
- ECU & Outputs · Powertrain
- Safe to drive?
- With caution
- Typical repair cost
- $20–$1,600
- DIY difficulty
- Moderate
P0628 means the PCM sees the voltage on its fuel pump control output lower than it should be for the commanded state – in practice, the wire or the load on it is connected to ground somewhere it shouldn't be. What you notice depends on how the PCM switches the relay. If the PCM grounds the relay coil, a short to ground energizes the relay permanently, so the pump runs whenever the key is on and never does its short two-second prime. If the PCM supplies the coil, the same short kills the output and the pump won't run at all, often with a blown fuse. A relay with a shorted coil or the wrong relay type can mimic both. Because a grounded wire can keep blowing fuses, find the short rather than fitting larger fuses.
Can I keep driving? If the pump runs normally and the engine starts, drive gently and get it checked soon. If fuses keep blowing, don't install a larger one – a grounded wire can overheat – and have the car towed if it won't start.
How to read P0628
- P Powertrain
- 0 Generic (SAE)
- 6 Computer & outputs
- 28 Specific fault
Symptoms
- Fuel pump keeps humming for as long as the key is on instead of stopping after about two seconds
- No start because the pump never switches on
- Fuel pump fuse or relay-feed fuse blows repeatedly
- Stalling on bumps as a chafed wire touches metal
- Check engine light with no drivability issue at all on some vehicles
Possible causes
Ranked by how often each cause is behind this code (editorial estimate).
- Control wire chafed to ground under carpet, a seat rail or along the frame
- Relay coil shorted internally, drawing far more current than normal
- Water in the fuse or relay box bridging the control terminal to ground
- Shorted input on the fuel pump driver module
- Wrong relay type fitted, for example one with a diode that doesn't suit the circuit
- PCM output driver shorted
Diagnosis step by step
Listen to the key-on prime
Switch the key on without cranking and time the pump. A normal system primes for roughly two seconds and stops. A pump that runs continuously points to a grounded control wire on a PCM that switches the relay's ground side; complete silence plus a blown fuse points to a short on a supply-side design.
Tools: OBD-II scanner
Check the relay coil and part number
Pull the relay and measure across the coil: most read about 60–120 ohms. Under roughly 40 ohms means shorted turns. Compare the part number with the original – relays with a built-in diode or resistor are not always interchangeable.
Tools: Multimeter
Test the control wire for a short to ground
With the relay out and the PCM connector unplugged, measure from the relay socket's control terminal to a clean chassis ground. It must read open (over several thousand ohms). A low reading confirms the short; unplug intermediate connectors to narrow down the section.
Tools: Multimeter
Inspect the relay box for water
Look for rust, white deposits or water marks in the fuse box, especially where it sits below the cowl, battery tray or a leaking windshield. Dry, clean and seal it, then retest the short-to-ground reading.
Tools: Flashlight, Screwdriver
Walk the harness
Follow the wire under the carpet, along seat rails and to the tank or pump module, looking for bare copper and melted insulation. Aftermarket seat covers, floor mats with clips and previous interior repairs are frequent culprits.
Tools: Flashlight
Confirm the PCM driver survived
After fixing the short, reconnect everything and watch the control pin with a meter or test light during key-on prime. A clean switch on and off shows the output is healthy; no switching means the PCM driver was damaged by the short.
Tools: Multimeter, OBD-II scanner
Where the parts are
- 1Battery
- 2Alternator
- 3Engine computer (ECM)
- 4CAN bus
- 5Transmission
- 6Wheel speed sensor
- 7High-voltage battery
Repair options & costs
| Repair | DIY? | Estimated cost |
|---|---|---|
| Replace relay with the correct type | Yes | $20–$80 |
| Repair control wire chafed to ground | No | $120–$380 |
| Dry, clean and reseal a wet fuse box | No | $100–$300 |
| Replace fuel pump driver module | No | $180–$500 |
| Replace PCM with programming | No | $700–$1,600 |
Rough US estimates for parts and labor; shop rates vary by region. Not a quote.
Emissions / smog test impact
Will fail
With P0628 stored the check engine light is on and the car fails an OBD smog check. Repair the short, clear the code and let the readiness monitors finish before the test.
Frequently asked questions
Why does my fuel pump run nonstop with the key on?
On many cars the PCM switches the relay coil to ground to turn the pump on. If the control wire touches ground, the relay stays energized and the pump runs whenever the key is on, which is one of the clearest signs of P0628.
Is it dangerous if the pump keeps running?
The pump runs only while its relay has power, which on most cars ends when you switch the key off. The concern is that the normal prime-and-stop safety logic no longer works, so a stalled engine or an accident won't shut the pump off as designed. Repair it promptly.
Can a wrong relay really cause this code?
Yes. Relays with an internal diode are polarity-sensitive, and some have a much lower coil resistance. Fitted in the wrong circuit they can draw excessive current or pull the control line down, which the PCM reports as circuit low.
What's the difference between P0628 and P0629?
P0628 means the control line is pulled toward ground; P0629 means it is being pulled toward battery voltage. They need opposite tests – a check to ground here, a search for a live wire there.
Sources
- SAE J2012 – Diagnostic Trouble Code Definitions
- ISO 15031-6 – Road vehicles, diagnostic trouble code definitions (EOBD)
- Manufacturer public service information (cross-checked)
- Methodology: How We Build Code Pages · Data Sources We Use · Editorial Policy