What it means
The control module detects a circuit-low condition in the Fuel Pressure Regulator A control circuit.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
- The engine or gearbox may go into limp mode
How it works
Fuel Pressure Regulator A controls fuel pressure by changing how much fuel reaches or returns from the fuel rail. The control module monitors the electrical circuit that operates Regulator A and sets P0091 when its signal is lower than expected, often because the circuit is pulled toward ground.
Common causes
Most likely first.
- Wiring shorted to ground
- Connector pins corroded - high contact resistance
- Harness chafing through insulation
Symptoms
- Check engine light on
- Hard start, no start, or stalling under load
- Power loss especially at high load or rpm
- Possible long cranking time
How to diagnose it
- Check freeze-frame data and fuel-pressure readings to see when the fault occurred and whether pressure control was affected.
- With the ignition off, inspect Regulator A's connector and nearby harness for corrosion, damaged pins, or insulation rubbed through to ground.
- Check the connector fit and terminal condition for corrosion or poor contact that could pull the control signal low.
- Use the wiring diagram to test the Regulator A control wire for a short to ground and check circuit continuity as specified by the manufacturer.
- If wiring and connections pass, test Regulator A and its driver circuit using the manufacturer's procedure before considering a control-module fault.
Common fixes
- Repair chafed wiring that is shorting the Regulator A control circuit to ground.
- Clean or repair corroded connector terminals and restore a secure connection.
- Replace damaged harness sections or terminals.
- Replace Fuel Pressure Regulator A only if testing confirms it is faulty.
Can I keep driving?
Avoid hard acceleration or heavy loads because incorrect fuel pressure can cause power loss or stalling. If the engine stalls, will not start, or fuel is leaking, stop driving and arrange a safe inspection.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.