Comprehensive diagnostic guide for P0629 Fuel Pump Control Circuit A High

What This Code Means

• Official definition (open-source code repository): P0629 - This indicates a high/input signal on the fuel pump control circuit A as seen by the vehicle's engine control module. Note: exact wording and naming can vary by manufacturer; the core idea is an abnormal or high condition on the fuel pump control circuit.
• Where it fits in OBD-II: P0629 is a Powertrain/OBD-II code. OBD-II codes are monitored by the powertrain control system (PCM) and used to diagnose issues related to engine performance and emissions. Wikipedia describes DTCs as monitors that trigger when issues are detected by the vehicle's electronic controls; powertrain codes are the subset dealing with engine/drivetrain systems (fuel system, ignition, emissions, etc.). This provides general framework for how a P0629-type fault is identified and stored .
• Quick takeaway: P0629 points to the fuel-pump control circuit (A) being signaled as high or out of expected range. The fault could originate in the wiring/sensors, the fuel pump circuit driver (FPDM/PCM), or the fuel pump/relay supply path.

Symptoms

• Engine may not start or may crank but fail to run.
• Intermittent no-start or stalling episodes.
• Engine may start but run poorly or stall if the FP circuit is intermittently signaling high or otherwise abnormal.
• In some cases, you might hear the fuel pump relay energizing or the pump running when not expected, depending on how the circuit is interpreted by the PCM.
• Symptom patterns to note for the diagnostic path:
  • Key-on engine-off: is there expected pump activity (if the PCM commands it) and does it stay on or go off?
  • During cranking: does fuel pressure build to normal levels?
  • After cranking attempts, does the PCM ever command the pump appropriately, or is the signal consistently high regardless of engine state?

Important safety and process notes

• Fuel system work is hazardous. Always relieve the fuel system pressure before disconnecting lines or servicing the pump/wiring. Work in a well-ventilated, non-spark environment; keep fire safety equipment accessible.
• Disconnect the battery if you're performing significant wiring repairs or replacing electronic control components to avoid short circuits or unintended pump energization.
• When testing, use proper PPE, and avoid open flames or hot surfaces near the fuel system.

Probable Causes

Note: The following probabilities are provided as practical guidance for diagnostic prioritization. They are not sourced from a single NHTSA dataset , so they reflect typical ASE/technician experience patterns.

• Wiring, harness, or connector problems in the fuel pump control circuit A (short to power, short to ground, open circuit, corrosion, damaged insulation): ~30%
• Fuel pump relay or its circuit being defective (coil failure, wiring to relay, poor connections): ~25%
• Fuel pump control module (FPDM) or PCM/ECU fault (internal driver issue, faulty output logic, or failed high-side/low-side drive circuit): ~20%
• Fuel pump itself or internal pressure/regulator issues causing abnormal circuit readings (fuel pump fatigued, clogged, or marginal performance): ~10%
• Power source or voltage concerns (low battery voltage, poor alternator output, excessive voltage drop in wiring): ~5%
• Intermittent faults or data interpretation issues (sensor data or wiring intermittently causing the PCM to read a "high" condition): ~10%

Step-by-Step Diagnosis

1) Confirm the DTC and initial data

• Use a capable scan tool to confirm P0629 is present and capture freeze-frame data: ignition status, engine RPM, fuel pressure (if the vehicle has live data), battery voltage, fuel trim, and any sensor readings at the moment the code was stored.
• Check for any related codes (e.g., other FP-related codes, circuit faults, or PCM/FPDM faults) that could point to a common root cause.

2) Basic power supply and ground checks

• Inspect fuses and relays related to the fuel pump circuit and FPDM/PCM supply. A blown fuse or sticky relay can create symptoms that mimic a high-input fault.
• Verify battery voltage at the vehicle battery and at the fuel pump circuit power supply path. Ensure voltage is present, stable, and within spec (typical 12-14V; note any abnormal voltage).
• Check ground integrity for the FPDM/PCM and the fuel pump circuit. A high-resistance ground can reflect as abnormal or high signals in certain fault conditions.

3) Fuel pump relay and circuit operation

• With key on (and engine off, if safe), check if the fuel pump relay coil energizes and if 12V is present at the fuel pump relay output/relays' switched side.
• If the relay operates normally, observe the fuel pump circuit voltage and current during cranking and running. If there is no current or inconsistent current, the fault may be in the wiring, connector, or FPDM/PCM drive.

4) Inspect the wiring harness for the fuel pump circuit A

• Visually inspect the wiring from the FPDM/PCM to the fuel pump connector, including harness runs near exhaust, harness conduits, and areas where chafing or corrosion can occur.
• Use a multimeter or scope to test continuity, resistance, and absence of shorts to battery or ground on the A circuit wire. Look for high resistance, open circuits, or impedance anomalies.

5) Inspect the fuel pump and FPDM/PCM drivers

• If wiring and relay checks are clean, test the FPDM/PCM signal:
  • Monitor the PCM's command signal to the FP circuit A in real time (live data). The PCM should command a normal pump-on signal only under appropriate conditions (key on, engine running, or cranking, depending on the design).
  • If the PCM indicates a high input on circuit A all the time or inappropriately, it can indicate a PCM/FPDM fault or a stuck/high-side driver.
• Check for internal FPDM/PCM faults if available (bench test, factory diagnostic procedure, or swap with a known-good module to confirm).

6) Fuel pressure and flow verification

• Connect a proper fuel pressure gauge and measure static (key on, engine off) and running fuel pressure. Compare with the vehicle's spec.
• If the pump is not delivering expected pressure despite correct signals, the problem might lie in the FP circuit control path, external regulation, or pump efficiency (though pump failure is less likely to produce a clear "A High" signal without additional symptoms).

7) Consider external data and environmental factors

• Check for other sensor data that could influence FPDM behavior (voltage supply to FPDM, status of ignition switch, or PCM fault codes). Some vehicles use the FPDM as a separate control module; in others, the FPDM logic is integrated into the PCM.
• If there are ECU/FPDM communication issues or a fault in the pump driver channel, the DTC may reflect a high signal condition rather than a pure pump failure.

8) If external faults are ruled out, plan the repair path

• Repair or replace any damaged wiring harness segments, connectors, or grounds.
• Replace a faulty fuel pump relay that shows signs of sticking or poor contact.
• If FPDM/PCM is suspected (or confirmed faulty after testing with a known-good unit), plan replacement or reprogramming as per vehicle-specific service information.
• If the fuel pump itself is marginal or failing to provide expected pressure, replacement may be needed after confirming the control circuit behavior and ensuring proper wiring.

8. Verification and post-repair steps

• Clear the DTCs and perform a road test or simulated operating conditions to verify that the code does not return.
• Re-check fuel pressure, voltage at the FP circuit, and FPDM/PCM drive signals after repair.
• Ensure no new codes appear and that the engine starts reliably without misfire or stalling.

Typical repair priorities (in order)

1) Wiring/connectors: clean up or replace damaged harness segments and fix grounds.

2) Fuel pump relay: replace if intermittent or seized; verify coil resistance and switching behavior.

3) FPDM/PCM drive path: confirm proper command signal to the fuel pump; repair/replace if faulty.

4) Fuel pump or pump circuit components: replace pump if it is marginal or failing to deliver adequate pressure.

5) Re-test and verify: clear codes, test drive, re-check for recurrence.

Notes on reference material

• Diagnostic codes overview: OBD-II DTCs are generated by the PCM as part of the vehicle's monitoring system, used for diagnosing issues in powertrain and emissions-related systems.
• Code definition source: The open-source code definition lists P0629 as (Fuel Pump Control Circuit A High). This is a direct phrasing from the provided open-source repository and indicates a high-level signal on the FP control circuit A.
• General vehicle diagnostic philosophy: The OBD-II framework uses DTCs to guide troubleshooting; the powertrain codes describe issues within engine and fuel system operations, including electrical and control circuitry.

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.

---