OBD Code P0629: Fuel Pump A Control Circuit High

When your vehicle’s onboard diagnostic system detects code P0629, it’s flagging a problem with how the engine control module (PCM) communicates with your fuel pump. Specifically, the voltage in the fuel pump control circuit is running higher than the normal operating range. This is an electrical issue that affects fuel delivery and can prevent your engine from starting or running properly.

What Does P0629 Mean?

P0629 stands for “Fuel Pump A Control Circuit High.” The code breaks down as follows:

• P = Powertrain system code
• 0 = Generic OBD-II code (applies to all manufacturers)
• 6 = Fuel system component
• 29 = Fuel pump A control circuit voltage too high

The fuel pump control circuit is the electrical pathway between your PCM and the fuel pump relay. The PCM sends a signal to activate the relay, which then supplies power to the fuel pump. When voltage in this circuit exceeds the expected range (typically above 5 volts for the control signal), the PCM stores this code.

This is different from a fuel pump electrical fault—it’s specifically about the control signal being too strong, which can damage the PCM’s output driver or prevent proper fuel pump operation.

Common Symptoms

• Engine won’t start or has difficulty starting
• Engine stalls while driving or at idle
• Loss of power during acceleration
• Fuel pump runs continuously or doesn’t run at all
• Check engine light illuminated
• Rough idle or hesitation when accelerating
• Fuel smell near the engine bay (if fuel leaks due to pump malfunction)
• No fuel pressure at the fuel rail

Possible Causes

Ranked from most to least common:

1. Faulty Fuel Pump Relay – The most common cause. A stuck or failing relay can send incorrect voltage signals to the PCM, triggering the high voltage code.
2. Damaged or Shorted Wiring – Corroded, pinched, or shorted wires in the fuel pump control circuit can cause voltage spikes that exceed normal operating parameters.
3. Poor Electrical Connections – Loose or corroded connectors at the fuel pump relay, PCM, or fuel pump can create resistance and voltage irregularities.
4. Faulty Fuel Pump Motor – A failing fuel pump with internal short circuits can draw excessive current, causing voltage fluctuations in the control circuit.
5. PCM Software Issue or Failure – In rare cases, outdated PCM software or a failing PCM can misinterpret normal voltage as “high” or fail to properly regulate the control signal.
6. Fuel Pump Relay Socket Corrosion – Corrosion inside the relay socket can prevent proper electrical contact and cause voltage anomalies.
7. Battery Voltage Regulator Problem – An overcharging alternator can cause system voltage to exceed normal ranges, affecting all circuits including fuel pump control.

Diagnostic Steps

Step 1: Scan and Document the Code

• Use an OBD-II scanner to confirm code P0629 and check for related codes (P0630, P0231, P0232, etc.)
• Note the freeze frame data, which shows engine conditions when the code was triggered
• Clear the code and test drive to see if it returns immediately

Step 2: Visual Inspection

• Locate the fuel pump relay (typically in the engine bay fuse/relay box—consult your service manual)
• Inspect the relay for signs of burning, corrosion, or physical damage
• Check all wiring and connectors related to the fuel pump circuit for damage, corrosion, or loose connections
• Look for pinched or rubbed wires that may be shorting to ground

Step 3: Check Battery and Charging System

• Test battery voltage with a multimeter (should be 12.6V at rest)
• Test charging voltage with the engine running (should be 13.5-14.5V)
• If charging voltage is above 15V, the alternator may be overcharging

Step 4: Test Fuel Pump Relay

• Remove the fuel pump relay from its socket
• Visually inspect the relay terminals for corrosion or damage
• Use a multimeter to test continuity across the relay coil and switch contacts
• Listen for an audible click when applying 12V to the relay coil (if testing manually)
• Swap the relay with an identical one from another circuit (like the horn relay) and retest

Step 5: Measure Control Circuit Voltage

• Set your multimeter to DC voltage mode
• Locate the fuel pump control wire at the relay or PCM connector
• Measure voltage with the key on and engine off (should be 0V or very low)
• Measure voltage with the engine cranking (should pulse between 0-12V)
• If voltage is consistently high or doesn’t return to 0V, suspect a PCM or wiring issue

Step 6: Check Fuel Pump Operation

• Listen for the fuel pump to prime when the key is first turned on (should run for 2-3 seconds)
• Use a fuel pressure gauge to measure fuel pressure at the fuel rail
• Normal fuel pressure is typically 35-65 PSI depending on the vehicle
• If pressure is low or zero, the pump may be failing

Step 7: Test PCM and Wiring Continuity

• Disconnect the PCM and fuel pump relay connectors
• Use an ohmmeter to test for continuity between the relay control pin and the PCM output pin
• Check for shorts to ground or power (resistance should be high for shorts to ground)
• If continuity is broken or shorts are found, repair or replace the wiring harness

Repair Cost Estimates

DIY Repair Costs:

• Fuel pump relay replacement: $15–$50 (parts only)
• Wiring repair or harness replacement: $20–$150 (materials)
• Connector cleaning/repair kit: $10–$30

Professional Repair Costs:

• Fuel pump relay replacement: $100–$250 (labor + parts)
• Fuel pump replacement: $400–$1,200 (labor + parts, varies by vehicle)
• PCM reprogramming or replacement: $500–$1,500 (labor + parts)
• Wiring harness repair: $200–$600 (labor + parts)
• Diagnostic fee: $75–$150 (often waived if you proceed with repairs)

Average Total Cost: $150–$400 for most cases (relay or wiring), up to $1,200+ if the fuel pump or PCM needs replacement.

Can I Still Drive?

Severity: Moderate to High

Whether you can safely drive with code P0629 depends on the specific symptoms:

• If the engine starts and runs normally: You may be able to drive short distances to a repair shop, but avoid highway driving. The fault could worsen suddenly.
• If the engine won’t start: Do not attempt to drive. You’ll need a tow truck.
• If the engine stalls while driving: This is a safety hazard. Do not drive the vehicle until repaired.
• If fuel pressure is low or zero: The engine will stall, leaving you stranded. Do not drive.

Recommendation: Have the vehicle diagnosed and repaired as soon as possible. Continuing to drive with this code risks engine stalling, which could cause an accident. In most cases, the repair is straightforward and affordable (relay replacement), so prioritize getting it fixed.

Frequently Asked Questions