A P0191 on the 6.7L Cummins means the fuel rail pressure sensor signal is outside its expected range, so the computer cannot trust the rail-pressure reading.
What P0191 means
Fuel Rail Pressure Sensor Performance is a diagnostic trouble code set when the monitored value falls outside its expected range. On the 6.7L Cummins, it narrows the problem to a specific system, and reading it correctly prevents replacing good parts.
Common causes
- Failed rail pressure sensor
- Corroded or damaged sensor wiring or connector
- Genuine rail-pressure problem the sensor is reporting
- Poor ground or reference voltage fault
Symptoms you may notice
- Power loss or hesitation
- Hard starting
- Check-engine light
- Possible rough running
How to diagnose it
Compare the sensor's reported pressure against commanded pressure and known-good data. A signal that is erratic or out of range with good wiring points at the sensor; verify the reference voltage and ground before replacing it.
How to fix it
Repair wiring and connector faults first, then replace the sensor if it has failed. If the sensor is accurately reporting a genuine pressure problem, diagnose the fuel-supply and high-pressure side instead. Clear and verify after the repair.
Frequently asked questions
Is P0191 the sensor or real pressure?
Compare reported versus commanded pressure. An erratic signal with good wiring suggests the sensor; an accurate low reading points to a real fuel-pressure problem.
Can it cause hard starting?
Yes. If the computer cannot trust rail pressure, fueling suffers and starting and running can be affected.
Check wiring first?
Yes. Corroded connectors and reference-voltage faults mimic a failed sensor, so inspect them before replacing the part.
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