Fault code P2635 – fuel pump “A” low flow/performance

Fault code P2635 is called “Fuel Pump (FP) “A” Low Flow/Performance” but in different programs it may be called differently. This fault designation applies to all vehicles equipped with OBD-II.

Technical description and explained code P2635

OBD-2 code P2635 is most often defined as “low fuel pump ‘A’ flow due to performance problems”. It is set when the PCM detects low fuel pump performance. It may be low flow rate or low fuel pressure.

Fault code P2635 – fuel pump

In most cases, the code is set immediately after the fault occurs and the signal lamp lights up. But sometimes the code is set only after the fault has been present for a few seconds.

On older gasoline engines, the fuel system is designed so that fuel flows to the injectors at a constant rate and pressure. In these systems, power is supplied to the fuel pump through a relay that supplies a voltage close to the battery voltage.

Modern vehicles use non-return systems that do not return excess fuel to the tank. Instead, the volume and pressure of fuel delivered to the injectors is controlled by varying the speed of the fuel pump, which is accomplished by dropping the voltage in the control circuit.

The downside of these systems, however, is the greatly increased complexity of fuel pump control. Which includes control modules and various sensors. Such as accelerator pedal position, throttle position. Inputs from MAP / MAF, coolant, intake air temperature, fuel pressure and several others. Which are interconnected via the Controller Area Network (CAN) bus system.

The PCM uses all these inputs to calculate the appropriate fuel pressure. To achieve the correct balance, the PCM adjusts the speed of the fuel pump several times per second by changing the input voltage. There are various ways to do this, but the most common method is to change the resistance to increase or decrease the voltage.

Since non-return systems use a variable voltage to control the speed of the pump. Therefore, it is more difficult to determine at what point in the operating range of the pump a failure has occurred.

However, in both designs, the PCM will set code P2635 and turn on the warning light, when low fuel pump performance is detected.

Symptoms of vehicle malfunctions

The main signal that an error P2635 has occurred is the Malfunction Indicator Lamp (MIL) is also known as the CheckEngine Light.

It can also be warning signs such as:

  1. The “Check engine” control lamp on the control panel will light up (the code will be memorized as a fault).
  2. Other related fault codes may also be present.
  3. The engine may fail to start or may start and then stall.
  4. Floating revolutions, and attempts to stall while idling.
  5. Insufficient fuel supply due to improper fuel pump operation.
  6. Reduced power output of the engine.
  7. Increased fuel consumption.
  8. Engine stalls when reaching operating temperature.
  9. There may be no symptoms other than a stored fault code.

A fuel pump malfunction can cause problems starting the engine, so error P2635 is considered serious.  If ignored, the fuel pump may fail completely.

Factors that can cause this error code

The error code P2635 can mean that one or more of the following problems have occurred:

  • Problems with the fuel pump itself.
  • Fuel pump relay is faulty and no voltage is supplied to the fuel pump.
  • Damaged wiring harness leads to breakage of fuel pump power supply circuit.
  • Loose or damaged connectors.
  • The feedback circuit is open.
  • A small impact may have tripped the inertia switch which opens the fuel pump circuit.
  • Fuel pump fuse blown due to short to ground.
  • Faulty CAN bus line.
  • Faulty engine control module (PCM).

How to fix or reset OBD-2 code P2635

Some suggested steps for troubleshooting and fix the error code P2635:

  1. Read all stored data and error codes with an OBD-II scan tool. To find out when and under what circumstances error P2635 appeared.
  2. Clear the error codes from the computer memory and test-drive the vehicle to find out if the fault appears again.
  3. Check that the fuel pump is working properly.
  4. Test the fuel pump relay.
  5. Visually inspect the electrical wires and connections for looseness and damage.
  6. Check the inertia switch, turn it on if necessary.
  7. Replace fuse if necessary.
  8. Clear the error code from the computer memory again, test drive the vehicle to see if the problem is resolved.

Diagnose and repair of problems

In order to diagnose the fuel pump circuits and eliminate the P2635 fault code, it is necessary to have a wiring diagram for your vehicle. After that you can proceed with further actions.

If the car starts and drives, no steering complaints, there is most likely an open circuit in the feedback circuit between the PCM and the fuel pump supply voltage circuit. Repair if necessary.

When the car won’t start or starts and shuts down because of low fuel pressure, check the fuel pump fuse. If it is blown, check the fuel pump supply circuit for a ground fault.

Remove the fuel pump relay (FP), and then check the resistance between the fuel pump connector voltage circuit and the ground. If there is a small resistance indicating a ground fault, disconnect the fuel pump and check it again.

If the fuel pump shows high or infinite resistance after disconnecting it, replace the shorted fuel pump. If the resistance is low after disconnecting the fuel pump, repair the short in the FP supply circuit.

After checking the electrical circuits, there will likely be several scenarios. Either the P2635 error will go away because the wiring problem has been fixed. Or the problem is a faulty relay or fuel pump.

On which vehicles does this problem occur most frequently

Fault code P2635 can occur on different vehicles but there are statistics on which brands this occurs most often. Here is a list of some of them:

  • Cadillac (Escalade)
  • Chevrolet (Captiva, Silverado, Tahoe)
  • Citroen (Dispatch, Jumper)
  • Ford (Transit)
  • GMC (Sierra)
  • Hummer (H3)
  • Jaguar (X-Type)
  • Opel (Antara, Astra, Insignia)
  • Peugeot (Boxer)

Fault code P2635 can sometimes be found with other errors. The most common are the following: P0627, P0628, P0629, P062A, P069E.

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