Fuel pump codes

How its codes differ

• P0171 and P0174 together can come from a weak pump or restricted supply, because every injector draws from the same fuel; trims that get worse under load point toward fuel delivery rather than an air leak.
• P0087 means the computer measured fuel rail or system pressure below its target. On direct-injection engines that can be the high-pressure pump, the tank pump feeding it, or the pressure sensor.
• P0300 can follow a supply shortfall, because a lean mixture under load can misfire on several cylinders.
• Rich codes (P0172, P0175) can come from fuel pressure that is too high, for example a faulty regulator or pressure control.

Codes

• P0171

  Engine computer has reached its limit adding fuel on bank 1: the mixture is lean

  Severity: Monitor

• P0174

  Engine computer has reached its limit adding fuel on bank 2: the mixture is lean

  Severity: Monitor

• P0300

  Random or multiple-cylinder misfire detected

  Severity: Inspect soon

• P0087

  Fuel pressure in the rail or supply system is lower than the engine computer requested

  Severity: Inspect soon

• P0172

  Engine computer has reached its limit removing fuel on bank 1: the mixture is rich

  Severity: Monitor

• P0175

  Engine computer has reached its limit removing fuel on bank 2: the mixture is rich

  Severity: Monitor

Signs it's failing

• Check engine light with lean codes or P0087
• Loss of power, especially at higher speed or under load
• Hard starting
• Stalling

How to test it

1. Note whether the problems are worst under load (uphill, accelerating, towing): a supply shortfall usually shows most when demand is highest. (Low risk)

2. Have fuel pressure and volume tested with a gauge against the vehicle's specification, including under load. This involves opening the fuel system. (Best left to a shop)

   Warning: The fuel system stays under pressure after the engine stops, and leaking fuel can cause injury or fire. Pressure relief and fuel line work belong in a shop.

After the repair

• Check for fuel leaks at every connection that was opened before driving.
• Recheck fuel trims under load.
• Clear codes only after the repair, then drive normally over several days with both city and highway driving so the readiness monitors can complete; clearing codes or disconnecting the battery resets them to not ready, which fails an emissions test.