P0191: Fuel Rail Pressure Sensor "A" Circuit Range/Performance
The fuel rail pressure reading does not fit what the engine control unit expects.
High risk
The engine may lose power or stall. Drive carefully and have it checked soon.
Short answer · Can I keep driving with P0191?
Only with care. The engine may hesitate, lose power or stall. Have it checked soon.
What does P0191 mean?
Code P0191 means the rail pressure signal is electrically valid but implausible, for example it does not change as expected or does not match the commanded pressure. A drifting sensor is common, but a real pressure problem can also cause it.
What the car does when the code is set
The check engine light comes on. Without a valid pressure reading, the engine control unit often limits power and uses a fixed fuel pressure. The engine may start hard, hesitate or stall, and on some engines it will not start at all.
How the system works
The fuel rail pressure sensor measures the pressure in the fuel rail that feeds the injectors. The engine control unit uses it to control the high-pressure pump or regulator and to calculate injection time. The sensor gets a 5 V reference and sends back a voltage that rises with pressure. A signal outside its normal range, or one that does not fit the other inputs, sets a sensor circuit code.
Common symptoms
Possible causes
Listed without ranking. The cause on your car depends on make, engine and mileage — see the maker notes below.
- Wiring or connector faultA chafed wire, corroded pin or loose plug at the rail pressure sensor disturbs the signal.
- Faulty rail pressure sensorThe sensor on the fuel rail can fail or drift with age.
- Missing 5 V reference or groundThe sensor shares its supply with other sensors; a short elsewhere can pull it down.
- Fuel leaking at the sensorA leak around the sensor thread or seal can damage the connector.
- Engine control unit input faultLess common, but the input that reads the sensor can fail.
How it is diagnosed
- Check for other sensor codesSeveral 5 V sensor codes at once point to a shared supply or ground problem rather than this sensor.
- Inspect the connector and wiringLook for oil or fuel in the plug, corrosion, bent pins and chafed wires along the rail.
- Compare the reading with the engine offWith the ignition on and the engine stopped, a workshop compares the reading with the expected rest value in the service data.
- Measure supply, ground and signalA workshop measures the reference voltage and signal at the connector before replacing the sensor.Fuel rails can hold very high pressure, especially on diesel and direct-injection engines; never loosen the sensor or lines without depressurising the system.
Questions about P0191
Can a real fuel pressure problem set P0191?
Yes. If the pressure itself is wrong, the reading will not fit. A workshop compares the sensor with a mechanical gauge where possible.
Can I replace a rail pressure sensor myself?
On many direct-injection and diesel engines the sensor is part of the rail or needs a precise torque and new seals. Because of the high fuel pressure, this is a job for a workshop.