P0191Powertrain
Powertrain · SAE
Rail Pressure Sensor — Range/Performance
Meaning
The engine control module has detected that the common-rail pressure sensor (RPS) signal is present but outside the expected range, does not correlate with the actual hydraulic pressure (measured indirectly), or its response to ECM commands is incorrect. The fault is not necessarily electrical but involves signal correlation.
Common causes
- Faulty rail pressure sensor with signal drift (systematically incorrect reading)
- Partially blocked measurement port on the sensor (deposits, sludge)
- Internal HP circuit leakage causing actual pressure to differ from commanded pressure
- Sensor wiring with abnormal resistance or corroded connector altering the signal
- Sensor supply voltage slightly out of specification (degraded 5 V reference)
- Interference on the signal circuit disturbing the pressure reading
Symptoms
- Check-engine light (MIL) illuminated
- Hard start or rough idle
- Misfires under acceleration and power reduction
- Abnormal exhaust smoke (reading too high: injection reduced; reading too low: excess fuel)
- Limp-home mode with fixed rail pressure
Severity
High: a rail sensor with incorrect readings compromises injection pressure regulation; the engine operates under non-optimal conditions with risk of injector damage.
What to do
- Compare the RPS reading with the rail pressure target at various RPM using live diagnostics
- Verify supply voltage (typically 5 V) and signal voltage at the sensor connector
- Check wiring continuity and resistance between the ECM and the sensor
- Replace the rail pressure sensor if wiring is serviceable and the reading is systematically incorrect
- After replacement, perform calibration/adaptation if required by the ECM