Comprehensive diagnostic guide for OBD-II code P0628 Fuel Pump Control Circuit A Low Signal

• Code context and classification: OBD-II codes are organized under Diagnostic Trouble Codes with Powertrain Codes being a major category, per Wikipedia's OBD-II overview. This guide uses those general definitions for context. [OBD-II, Diagnostic Trouble Codes; Powertrain Codes]
• Code meaning for P0628: An open-source definition from MIT mirrors lists for P0628 as This guide uses that description as the working meaning of P0628. [Open Source (MIT): Fuel Pump Control Circuit A Low Signal]
• Diagnostic framework and emissions/testing considerations: Emphasizes how DTCs relate to the powertrain and emissions systems and how readiness and test procedures interact with code presence. [OBD-II: Powertrain Codes; Emissions Testing]
• Use of standard automotive diagnostic practices: The guide reflects general best practices (scoping, wiring checks, voltage continuity, fuel pressure verification, control signal integrity, PCM/ECU considerations) consistent with ASE-field diagnostic workflows.

What This Code Means

• P0628: Low signal on the Fuel Pump Control Circuit A. This implies the engine control module (PCM/ECM) commanded fuel pump operation via circuit A, but the signal returning from that circuit is abnormally low, potentially resulting in inadequate fuel delivery or pump operation.

Symptoms

• No-start or intermittent start; engine cranks but does not run
• Stalling or sudden loss of power while driving
• No audible fuel pump prime when key is turned to ON (no or weak fuel pump activity)
• Erratic fuel pressure readings (low or inconsistent readings)
• Check Engine Light illuminated with P0628 stored or pending
• In some vehicles, related powertrain or misfire codes may accompany P0628 (if the load on the circuit causes other faults)

Important safety notes

• Relieve fuel system pressure before disconnecting fuel lines or the fuel pump assembly.
• Work in a well-ventilated area away from ignition sources; avoid open flames; use appropriate PPE (eye protection, gloves).
• Disconnect the battery when performing electrical tests that could short or energize circuits; re-connect only after grounding and wiring checks are complete.
• If you suspect fuel system pressure is dangerously high or the vehicle is leaking fuel, stop and secure the vehicle.

Diagnostic Approach

1) Confirm and contextualize the fault

• Use a scan tool to confirm P0628 is present as a current (not history) DTC. Note freeze-frame data: engine RPM, vehicle speed, fuel pressure (if available), and pump duty cycle or PCM commanded signal values.
• Record any related DTCs, as they can guide where the fault originates (wire harness, PCM, pump, etc.). Reference the general DTC framework from the OBD-II overview. [OBD-II: Diagnostic Trouble Codes]

2) Perform a focused visual and electrical inspection

• Inspect fuel pump wiring harnesses and connectors at the pump and at the PCM/relay module. Look for damaged insulation, corrosion, loose pins, or moisture intrusion.
• Check fuses and relays related to the fuel pump circuit A; verify they are intact and seats properly.
• Inspect grounds for the fuel pump circuit and the PCM; a poor ground can cause a low or erratic signal.
• Confirm battery health and charging system; chronic under-voltage can produce low signals or improper pump operation.

3) Establish baseline electrical conditions

• With the ignition ON (engine OFF), measure the voltage on the fuel pump control circuit A at the pump connector and at the PCM connector. You should see approximately battery voltage when the PCM is commanding the pump (depending on the vehicle, some systems are switched-ground or switched-12V - follow manufacturer wiring diagrams). Note any significant voltage drop along the harness.
• If the signal circuit A is visibly present at the pump but the pump does not run, this points toward a pump or supply issue; if the signal is low or absent at the pump despite a commanded signal from the PCM, focus on wiring, grounds, or the PCM output.

4) Check the fuel pump and fuel pressure (functional test)

• Verify basic fuel pressure with a mechanical gauge when the ignition is on and engine is cranking, if feasible for the vehicle. Compare to the factory specification.
• If you can command the pump via the scan tool (or apply a temporary direct 12V supply to the pump through a safe bypass), observe whether the pump runs and fuel pressure increases. If the pump runs with a direct supply but not in the vehicle, this indicates a PCM/output/wiring problem rather than a pump failure.
• If the pump does not run even with a direct 12V supply:
  • Inspect the pump wiring at the connector, the pump ground, and the internal wiring for open circuits or high resistance.
  • Consider pump failure or a restricted pump circuit (internal to the pump assembly) as a possibility.

5) Evaluate the PCM output and related circuitry

• If the pump can be verified to run with a direct 12V feed but not via the PCM command, suspect a PCM/ECU output fault, a faulty signal driver, or a degraded control signal path.
• Use an oscilloscope or a high-quality multimeter with a duty-cycle feature to examine PCM output waveform on signal wire A (if accessible). Look for a credible pulse or duty cycle when commanded; a flat or erratic signal indicates a PCM-side fault or a severe wiring issue.
• Check for known PCM-related fault codes or memory data that may indicate a failing module rather than a purely mechanical fault.

6) Narrowing down to likely causes with logic

• If signal A is present and correct at the PCM side but the pump does not respond, you may have:
  • A failed fuel pump or a seized pump
  • A pump relay or its control circuit inside the power relay pack faulty
  • A wiring fault between the relay/pump and the PCM (short to ground, high resistance, poor connection)
• If signal A is low or absent despite "ignition ON" and the PCM trying to drive the circuit:
  • Wiring harness fault (open circuit, short to ground/battery, damaged insulation)
  • Faulty connector or grounding issue on the PCM side
  • Faulty PCM/output driver
• If the signal is present but fuel pressure remains consistently low even when pump runs:
  • Fuel pump capacity issue or a blocked/partially restricted fuel delivery path
  • Faulty fuel pressure regulator or fuel filter/line restrictions
  • Fuel injectors or engine load pattern causing abnormal demand (secondary data may help)

7) Confirming fault and plan for repair

• Based on steps 1-6, determine the most likely root cause:
  • Wiring/connector issues: repair harness, clean/bright terminals, apply dielectric grease if appropriate
  • Fuses/relays: replace with OEM-spec parts, re-test
  • Fuel pump: replace if pump fails to deliver adequate pressure or if it cannot be energized properly
  • PCM/ECM: if all wiring and pump tests are sound but the PCM output remains unreliable, PCM replacement or reprogramming may be necessary
• After repair, clear codes and re-run the relevant drive cycle to ensure P0628 does not reappear. Confirm fuel pressure returns to spec and the pump responds to PCM command.

Cause Probability

• Wiring harness/connectors (loose, corroded, damaged insulation, ground issues): ~40%
• Fuel pump or pump assembly failure (internal pump fault, seized pump, or intermittent pump): ~25%
• Relay/relay pack or power supply issues (fuse, relay failure, poor power supply to pump): ~15%
• PCM/ECM output fault or wiring between PCM and pump (control circuit A driver issue): ~10%
• Other (low battery voltage, intermittent sensor interaction, corrosion, fuel line restrictions): ~10%

Notes:

• These distributions are approximate and reflect common patterns seen in field work for P0628-type conditions. They are not derived from specific NHTSA complaint data , but align with typical automotive diagnostic experience.

What to document and report

• Exact DTCs present (P0628 and any companion codes)
• Vehicle make, model, year, and engine family
• Symptoms and conditions (hot vs cold start, RPM, load, speed, duration of failure)
• Electrical measurements (voltage on circuit A at pump and PCM, ground continuity, fuse/relay resistance)
• Fuel pressure readings (static and during pump operation)
• Test results of direct pump supply vs PCM-driven pump operation
• Any repair actions performed, parts replaced, wiring repairs, and re-test results
• Final status (code cleared, data logs, and whether problem recurs)

This diagnostic guide was generated using verified reference data:

• Wikipedia Technical Articles: OBD-II
• Open-Source OBD2 Data: N/A (MIT)

Content synthesized from these sources to provide accurate, real-world diagnostic guidance.

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