Fuel and air

P01A4 — Alternative Fuel Rail Pressure Sensor Performance

Severity: medium Drive with care, to a shop

What P01A4 means

The engine control module compares the signal from the alternative (secondary) fuel rail pressure sensor with the commanded pressure and with the primary sensor. If the signal doesn't match engine conditions, drifts out of range or gets stuck, P01A4 is stored. The cause can be the sensor or its wiring, or a real fuel pressure problem.

Symptoms

  • Check Engine light on
  • Rough or unstable idle
  • Hesitation or stumbling when accelerating
  • Reduced power, especially under load
  • Long cranking or hard starting
  • Worse fuel economy
  • Possible stalling

Possible causes

  1. Failed fuel rail pressure sensor
  2. Damaged wiring or corroded connector at the sensor (open, short, poor ground)
  3. Low pressure in the low-pressure circuit: fuel pump, filter or lines
  4. Worn or failing high-pressure fuel pump
  5. Faulty pressure regulator or volume control valve
  6. Excessive internal leakage (injector return flow too high)
  7. Rarely, an ECM fault

How to diagnose

  1. Scan for codes and freeze-frame data, and look for other fuel pressure codes
  2. Compare specified and actual rail pressure at cranking, idle and under load
  3. Compare the alternative sensor reading with the primary sensor
  4. Inspect the sensor, connector and harness for corrosion, chafing or loose pins
  5. Check sensor power, ground and reference voltage with a multimeter
  6. Test low-pressure supply pressure and volume, and check the fuel filter
  7. Check the regulator or volume control valve operation
  8. Measure injector return flow if applicable

Common fixes

  • Replace the fuel rail pressure sensor
  • Repair or replace damaged wiring and connectors
  • Replace the fuel pump or filter if supply pressure is low
  • Replace the pressure regulator or volume control valve
  • Repair or replace the high-pressure pump or leaking injectors

Questions and answers

What does P01A4 mean?

The ECM found that the alternative fuel rail pressure sensor reading is implausible or doesn't match what's expected. The cause may be the sensor, its wiring, or a real fuel pressure problem.

Can I drive with P01A4?

Only briefly and carefully, to get to a shop. If fuel pressure is truly low, the engine may lose power or stall while driving.

Will replacing the sensor fix it?

Not always. Compare specified and actual pressure and check the wiring and fuel supply first, otherwise a new sensor may not solve it.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.