Fuel pressure regulator control solenoid or actuator with an internal short or failed winding; Control wiring shorted to ground or chafed, damaged insulation between the regulator and the ECU; Corroded, loose or contaminated connector terminals at the regulator or ECU; Faulty ECU output driver for the regulator; Wiring harness damage from heat, vibration or previous repair work; Low or unstable supply voltage to the regulator circuit, such as a blown fuse or poor earth.

P0091

Fuel Pressure Regulator A Control Circuit Low.

Generic
Powertrain
Generic code
Medium severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
0: AreaFuel and air metering, auxiliary emission controls
91: FaultThe specific fault in that area

What it means

P0091 is a generic powertrain code meaning the engine control unit has detected a low voltage or low signal state in the control circuit for fuel pressure regulator A. The ECU monitors the circuit that drives the regulator and sets the code when the voltage or current is lower than expected, which usually points to a short to ground or a fault in the driver circuit rather than a fuel pressure fault itself.

Symptoms

Malfunction indicator lamp on
Hard starting or long cranking
Rough idle or stalling
Hesitation or loss of power under load
Poor fuel economy
Engine may run in a limp mode on some vehicles

Possible causes

Fuel pressure regulator control solenoid or actuator with an internal short or failed windinglikely
Control wiring shorted to ground or chafed, damaged insulation between the regulator and the ECUlikely
Corroded, loose or contaminated connector terminals at the regulator or ECUpossible
Faulty ECU output driver for the regulatorrare
Wiring harness damage from heat, vibration or previous repair workpossible
Low or unstable supply voltage to the regulator circuit, such as a blown fuse or poor earthpossible

How to diagnose

Read all stored codes and freeze frame data, noting engine load, temperature and rail pressure at the time the code set, and check for related fuel pressure codes
Visually inspect the regulator, its connector and the wiring for damage, chafing, corrosion, fuel contamination or water ingress
Check the relevant fuses, relays and power and earth supplies to the regulator circuit using the manufacturer wiring diagram
Measure the regulator coil resistance and compare it with the specification for that engine, as the layout and values depend on the model and fuel system type
Test the control wire for a short to ground and for continuity back to the ECU with the connectors disconnected
With a suitable scan tool, watch commanded versus actual fuel pressure and, where supported, activate the regulator to check it responds
Check the ECU driver signal with a scope or test lamp as the procedure specifies for that vehicle
Check for any technical bulletins or calibration updates for that model if the wiring and regulator test good

How to fix

Replace the fuel pressure regulator or its control solenoid if the winding or operation is out of specification
Repair or replace damaged wiring and secure the harness away from heat and abrasion
Clean or repair corroded connector terminals, or replace the connector if the pins are damaged or loose
Restore any missing power or earth supply and replace blown fuses once the cause of the overload is found
Replace or repair the ECU only after wiring and regulator faults have been ruled out, and then follow the manufacturer procedure
Apply a manufacturer calibration update only where an outdated calibration is a documented cause for that model

Can I keep driving?

Medium severity

The car may be driveable for a short time, but fuel pressure control may be poor, so expect rough running, stalling or reduced power. Get it checked soon and avoid long trips or overtaking situations where a sudden loss of power would be unsafe.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.