Symptoms of P0191
- Check-engine light on, sometimes with rough or uneven running.
- Hesitation, surging, or a noticeable change in fuel economy.
- Occasionally hard starting or a smell of unburnt fuel.
Common causes of P0191
- A failing or contaminated sensor (MAF, MAP, oxygen or air-fuel sensor).
- Intake or vacuum leaks, or an exhaust leak near an upstream sensor.
- Wiring, connector, or fuel-delivery problems that skew the reading.
Useful checks before replacing parts
- Compare commanded rail pressure, actual pressure, low-side pressure, pump command, regulator duty, engine load, and fuel temperature.
- Inspect sensor response, connector condition, wiring resistance, fuel restriction, pump output, regulator behavior, and pressure decay.
- Check whether the performance fault appears during cranking, idle, acceleration, high load, or hot restart.
Related OBD-II codes
These nearby fuel / air entries can help compare wording and pin down the fault area.