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Error Code P0629 is defined as Fuel Pump A Control Circuit High. This means there’s a problem with the “A” control circuit in the fuel pump, which is caused by bad connectors/wires or defective CAN bus.
This code is a generic trouble code, meaning it applies to all vehicles equipped with the OBD-II system, particularly those made since 1996 up to present. It’s more common among Chevrolet, Chrysler, Dodge, Ford, Mitsubishi, Nissan, Jeep, Toyota, etc. Specifications on the definition, troubleshooting, and repairs, of course, vary from one make and model to another.
When Error Code P0629 appears, that means there’s a problem detected in the fuel pump “A” control circuit. This specifically means there’s a high voltage situation problem detected by the PCM (powertrain control module, also known as ECM or engine control module in other vehicle makes). Other supporting modules can also spark up the particular code, such as:
- Alternative Fuel Control Module
- Fuel Injection Control Module
- Turbo Control Module
Depending on the make and model of the vehicle, it may take multiple drive cycles before the fault code is activated. In other cases, it can be an immediate reaction as soon as the PCM recognizes the fault.
The fuel pump plays an integral role in the vehicle’s overall drivability. Obviously, without it, there will be no fuel being delivered to the engine. Depending on the needs of the driver, the control circuit is responsible for turning the pump on and off. An open condition within this circuit could also cause this code, so take note of this before proceeding with any diagnosis.
Other related codes include:
- Engine no start condition or starts but dies
- Engine cranks but doesn’t start
- Engine misfire or stalling
- Increase in fuel consumption
- Engine dies when reaching a normal operating temperature
It is still possible for the problem to remain unresolved even the Check Engine light doesn’t light up immediately. Thus, it’s always important to make sure the vehicle has gone through multiple drive cycles (driving the vehicle for a week. If the Check Engine light lights up, then the problem remains.
- Defective fuel lamp
- Damaged or severed ground wire in driver’s control module
- Corroded, open, or shorted wiring in the CAN bus
- Unattached ground strap in the control module
- Defective CAN bus
- Unsecured wiring harness and wires causing broken or chafe circuit
- High resistance in the circuit (corroded or melted connectors, internal wire corrosion)
How to Check
As with most codes, it is best to check with the TSB (Technical Service Bulletin) when diagnosing for this code. Check with the vehicle’s specific year, model, and power plant. This can save a lot of time in many circumstances.
Scan and test each module using the OBD-II scanner to get a good idea of the electrical condition of the vehicle’s modules. Do a visual inspection of the connectors and wirings. Anything that is obviously damaged must be repaired or replaced, as necessary. Many times this is located under the vehicle, near the fuel tank. This part is susceptible to road debris and other elements, so make sure you pay attention to the condition of these parts.
When dealing with any component in its own module (fuel pump module, etc.), make sure you check the ground circuits. You can do this using separate battery ground. You can also easily do this using auxiliary ground cable.
If the problem is fixed using the auxiliary ground attached but comes back when using OEM ground, then the problem is in the ground cable. It must be repaired or replaced. Grounds must be thoroughly inspected for corroded connections. Terminal pins, etc., can also cause the resistance of the circuit. A sign of excessive corrosion a green ring around the connector that is attached to the positive battery terminal. If this is present, then remove the terminal and make sure you clean all contact points, connector face, and terminal post/stud.
Because an open circuit could cause this code, it’s important to identify the circuit using the electrical wiring diagram from your service manual. Then, trace the individual fuel pump A control wire individually to check for obvious signs of damages in the wire. Repair as necessary either by soldering the wire, or using butt connectors with a heath shrink to insulate it from various elements.
Then, use the multimeter to measure the resistance between the connectors in the circuit to pinpoint the location of the short/open. If there is a fault somewhere inside the whole circuit, it is highly recommended to use a power probe style tool.
How to Fix
Depending on the diagnosis, common repairs for this code include:
- Replacement of defective fuel pump or fuel pump relay
- Repair or replacement of any electrical components in the CAN bus harness, such as corroded, open, or shorted connectors, or wirings
- Repair or replacement of fuel pump wiring harness
- Replacement of defective fuel pump
- Replacement of broken control module ground wire
- Adjustment of loose control module ground strap
- Replacement of defective CAN bus
- Repair or replacement of electrical connections in the fuel pump circuit
Though the vehicle can still be driven with this error code present, it is highly discouraged, as fuel may be intermittently delivered to the engine. Erratic of fluctuating mixture of fuel could result in serious engine damage.