Fuel and air

P0091 — Fuel Pressure Regulator 1 Control Circuit Low

Severity: medium Drive with care, to a shop

What P0091 means

The PCM controls a fuel pressure regulator (on many diesel and direct-injection engines, a metering valve at the pump or fuel rail) and monitors the voltage in its control circuit. P0091 is set when that signal falls below the expected range, for example from a short to ground, an open circuit or a failed regulator. Rail pressure then can't be held accurately.

Symptoms

  • Check Engine light on
  • Hard or long cranking before starting
  • Rough idle
  • Stalling, especially under load
  • Reduced power or hesitation
  • Worse fuel economy
  • Possible low fuel pressure codes

Possible causes

  1. Failed fuel pressure regulator (metering valve)
  2. Damaged, chafed or corroded wiring and connector terminals
  3. Control wire shorted to ground
  4. Faulty fuel rail pressure sensor
  5. PCM output driver fault
  6. Low supply pressure from a weak fuel pump or clogged filter

How to diagnose

  1. Scan all codes and freeze frame data, and note any related fuel pressure codes
  2. Compare commanded and actual rail pressure at cranking, idle and under load
  3. Test low-pressure supply pressure and volume (pump, filter)
  4. Inspect the regulator and rail sensor connectors for corrosion, moisture and loose pins
  5. Test the control wire for a short to ground and for opens
  6. Measure regulator coil resistance and compare to specification
  7. Command the regulator with a scan tool active test and watch the response
  8. Check the rail pressure sensor and, where applicable, injector return flow

Common fixes

  • Repair wiring and connectors
  • Replace the fuel pressure regulator (metering valve)
  • Replace the fuel pressure sensor
  • Fix low supply pressure by replacing the filter or fuel pump
  • Reflash or replace the PCM if its output is faulty

Questions and answers

What does P0091 mean?

The PCM detected low voltage in the control circuit of fuel pressure regulator 1. Wiring, a connector or the regulator itself is the most common cause.

Can I drive with P0091?

Only briefly and carefully to a shop. The engine can stall or lose power, which is dangerous in traffic.

How do I fix P0091?

Start with the regulator wiring and connector, then check rail pressure and the regulator. Replace the PCM only as a last resort.

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.