What it means
The fuel rail pressure sensor signal is within electrical limits but doesn't match commanded rail pressure under various engine loads. Common in common-rail diesel and GDI petrol engines.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
Fuel rail pressure sensor A reports rail pressure to the engine control module, which compares the reading with the pressure it commands as engine demand changes. P0191 sets when the signal stays within electrical limits but the reported rail pressure does not track the commanded pressure as expected.
Common causes
Most likely first.
- Pressure sensor drifted
- Pressure regulator stuck
- High-pressure pump worn
Symptoms
- Check engine light on
- Hard starting or extended cranking
- Rough running or stalling
- Reduced engine power
How to diagnose it
- Review freeze-frame data and graph live rail pressure sensor A against commanded rail pressure during cranking, idle, and changing engine load.
- Inspect sensor A's connector, mounting, and wiring for looseness, corrosion, or damage that could distort the pressure reading.
- Check the sensor A supply, ground, and signal circuits for stable connections and compare them with the manufacturer's specification.
- Use the manufacturer's procedure to verify actual rail pressure independently and determine whether the discrepancy is in the reading or fuel delivery.
- If actual pressure fails to follow command, test the pressure regulator and high-pressure pump; if actual pressure is correct, test sensor A for drift.
Common fixes
- Repair loose or damaged wiring and connector contacts at rail pressure sensor A.
- Replace sensor A if it drifts outside the manufacturer's specification while actual pressure is correct.
- Repair or replace a sticking pressure regulator if it fails testing.
- Repair or replace the high-pressure pump if testing confirms it cannot maintain commanded rail pressure.
Can I keep driving?
Avoid hard acceleration and long trips if the engine is cranking slowly, running roughly, or losing power. Stalling can become a traffic hazard, and continued operation with incorrect rail pressure can worsen drivability.
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.