When the PCM detects a low‑voltage condition on the primary fuel‑pump control line, the Check Engine Light illuminates with the P0628 code. Drivers typically report:
* Hard start or delayed crank‑to‑run – the engine may turn over normally but take several seconds to fire.
* Engine stall at idle or under light load – the vehicle can die while stopped at a traffic light or during gentle acceleration.
* Loss of power during acceleration – a sudden dip in RPMs or hesitation when the throttle is opened quickly.
* Intermittent fuel‑pump operation – the pump may cut out briefly, causing a momentary surge in fuel pressure.
These signs appear when the PCM limits fuel‑pump duty to protect the circuit from a perceived low‑voltage condition.
The PCM includes an internal regulator that supplies the fuel‑pump driver. If the regulator can no longer maintain the required 12 V, the PCM records a low‑circuit condition and sets P0628. This failure is internal to the control module and cannot be corrected by external components.
A broken conductor, cracked insulation, or a short to ground in the fuel‑pump control wire will drop voltage below the PCM’s threshold. The fault may be localized to a connector pin, a section of harness near the fuel tank, or a splice that has become loose.
The fuel‑pump driver relies on a solid chassis ground. Rust, paint, or loose bolts can increase resistance, causing the PCM to read a low voltage. Even a few ohms of added resistance can be enough to trigger the code.
The driver transistor that switches the pump on and off can fail open or develop high leakage. When the MOSFET cannot deliver full voltage, the PCM interprets the condition as “low circuit” and logs P0628.
Heavy accessory loads (e.g., high‑current lighting, aftermarket audio) that share the same power distribution can momentarily sag the supply to the fuel‑pump driver. If the sag is frequent, the PCM may flag a low‑circuit condition.
* Connect a professional scan tool capable of reading PCM data. Confirm P0628 and note any accompanying codes (e.g., B1234 for ground issues) that may point to a broader wiring problem.
* Examine the fuel‑pump control harness from the PCM to the pump. Look for chafed wires, cracked insulation, burnt pins, or loose terminals. Clean any corrosion with a proper electrical contact cleaner and re‑torque connectors to manufacturer torque specs.
* With the ignition ON (engine off), measure voltage at the PCM’s fuel‑pump control output pin. Expected reading is close to battery voltage (≈12.5 V).
* Activate the pump by turning the key to the “run” position; voltage should remain stable. A drop below 9 V indicates a circuit problem.
* Measure ground resistance at the pump’s chassis ground point; values above 0.5 Ω suggest a poor ground.
* If wiring and grounds are sound, the fault likely resides in the PCM. Using a scope or a high‑impedance voltmeter, monitor the driver signal while the engine attempts to start. Absence of a proper PWM signal confirms a PCM internal failure.
* Run a bidirectional test via the scan tool to verify the PCM can send and receive data on the CAN bus. Communication errors may indicate a need for re‑programming or replacement.
* Wiring/ground repair – Replace damaged sections, reseat connectors, and secure grounds. Re‑test the voltage; most low‑circuit codes resolve if the circuit integrity is restored.
* PCM re‑programming – If the driver voltage is borderline, a software update may recalibrate the voltage thresholds. Re‑flash the PCM using the manufacturer’s latest calibration file.
* PCM replacement – When the driver MOSFET or internal regulator is defective, replacement is the most reliable solution. Ensure the new unit is VIN‑matched and programmed with the vehicle’s calibration data.
* Wiring repair and connector cleaning: $50‑$150 for parts and labor.
* PCM re‑programming (software update only): $80‑$120 labor.
* PCM replacement (including VIN‑matched programming): $600‑$900 for the module plus $200‑$300 labor.
* Regular connector inspection – Every 12 months, check the fuel‑pump control connector for moisture, corrosion, or looseness. Apply dielectric grease to protect against future oxidation.
* Ground integrity – Keep chassis ground points clean and free of paint. Tighten grounding bolts to the specified torque during scheduled service.
* Electrical load management – Avoid adding high‑current accessories that draw from the same power distribution without upgrading the alternator or wiring gauge.
* PCM software updates – When the manufacturer releases a PCM calibration update, have it applied during routine service to ensure the latest voltage‑monitoring algorithms are in place.
Service Recommendation: Most issues related to this fault are diagnosed and corrected through inspection, wiring repair, and calibration rather than module replacement. For modules not typically replaced through aftermarket suppliers, diagnosis and repair should be performed by a certified automotive technician with access to factory service information and tooling.