What this code means
P0191 may mean the engine controller is seeing a fuel rail pressure sensor performance problem, often because the pressure readings do not agree the way the controller expects.
What the vehicle may do
- The truck may run normally with only a warning light.
- It may have possible drivability symptoms if fuel rail pressure information is unreliable.
- It can be intermittent when wiring or connector contact changes with vibration.
Possible fault areas
- Possible fuel rail pressure sensor issue.
- Possible signal circuit issue.
- Possible reference voltage or low reference issue.
- Possible connector, terminal, or harness issue.
- Possible engine control module involvement after circuit checks pass.
Diagnostic path
Opening context
On this Silverado diesel, P0191 is a fuel rail pressure sensor performance code. In plain language, the vehicle may be seeing a mismatch between the two fuel rail pressure sensor readings. The truck may run normally, or it may have possible drivability issues if the rail pressure information is not believable. Broadly, this can involve the fuel rail pressure sensor, the signal circuits, the reference voltage, the low reference, connectors, wiring, or the engine control module. If other codes are present, check what they mean first, then start with the basic system checks and follow a structured diagnostic approach before getting into this P0191 path.
What P0191 is checking
The fuel rail pressure sensor measures pressure in the fuel rail. The control module supplies the regulated reference voltage, reads the signal circuits, and provides the low reference ground path. In the scan data definitions, Fuel Rail Pressure Sensor 1 has a parameter normal range of 1 to 249 MPa (145 to 36,114 PSI), and Fuel Rail Pressure Sensor 2 has the same parameter normal range of 1 to 249 MPa (145 to 36,114 PSI). The failure-mode displayed values matter too: Fuel Rail Pressure Sensor 1 can show 250 MPa (36,259 PSI) for a short, open, or short-to-voltage condition, and 0 MPa (0 PSI) for a signal short-to-ground condition. Fuel Rail Pressure Sensor 2 can show 250 MPa (36,259 PSI) for a signal short-to-ground condition, and 0 MPa (0 PSI) for a 5 V reference, open, or short-to-voltage condition. For P0191, treat the enabling conditions as monitor gates, not as the repair by themselves. The useful operating gates are ECT Sensor greater than −40°C (−40°F), Engine Off Time greater than 5 min, ignition on, Ignition Voltage greater than 11 V, and the related enabling faults not being set. The key setting rule is sensor agreement: Fuel Rail Pressure Sensor 1 and Fuel Rail Pressure Sensor 2 are not within 21 MPa (3046 PSI) of each other for greater than 1 s.
Circuit and system verification
Start the verification with the engine idling. On the scan tool, both Fuel Rail Pressure Sensor 1 and Fuel Rail Pressure Sensor 2 should read 1 to 249 MPa (145 to 36,114 PSI). If they are not in that range, move into circuit testing. If they are in range, command Fuel Rail Pressure Increase and Decrease with the scan tool. Both values should change with each command. If the values do not respond, move into circuit testing. If they do respond, compare the two readings. They should be within 21 MPa (3046 PSI) of each other. If they are not, move into circuit testing. If they agree, watch the pressure readings while wiggling the harness and connectors at the B47B Fuel Rail Pressure Sensor and at the K20 Engine Control Module. The parameter value should not change. If it changes during the wiggle test, repair the wiring, terminals, or electrical connectors as needed. If it stays stable, reproduce the operating conditions for the code, or the captured conditions from the earlier failure data, and verify P0191 does not reset. If it does not reset, the check is OK at this time. If it resets, continue into circuit testing.
Low reference circuit testing
For circuit testing, turn the ignition and all vehicle systems off. It may take up to 2 min for all vehicle systems to power down before an accurate ground or low reference continuity test can be made. Disconnect the B47B Fuel Rail Pressure Sensor connector. Test between Low Reference circuit terminal 3 and ground. You want less than 10 Ω. If the reading is 10 Ω or greater, disconnect the appropriate K20 Engine Control Module connector and test the low reference from terminal 3 at the component harness to the other end of the circuit at the control module harness. That circuit should be less than 2 Ω. If it is 2 Ω or greater, repair the open or high resistance in that circuit. If it is less than 2 Ω, that path points to the K20 Engine Control Module.
5 V reference testing
If the low reference to ground test was less than 10 Ω, turn the ignition on with the vehicle in Service Mode. Test between the 5 V Reference circuit terminal 1 and Low Reference circuit terminal 3. The correct range is 4.8 to 5.2 V. If it is less than 4.8 V, turn the vehicle off, disconnect the appropriate K20 Engine Control Module connector, and test from 5 V Reference circuit terminal 1 at the component harness to ground for infinite resistance. Anything less than infinite resistance means repair the short to ground. If it is infinite resistance, test the 5 V reference circuit from the component harness to the control module harness. That should be less than 2 Ω. If it is 2 Ω or greater, repair the open or high resistance. If it is less than 2 Ω, that path points to the K20 Engine Control Module. If the original 5 V reference test is greater than 5.2 V, turn the vehicle off, disconnect the appropriate K20 connector, turn the ignition back on in Service Mode, and test the 5 V reference at the component harness to ground. It should be less than 1 V. If it is 1 V or greater, repair the short to voltage. If it is less than 1 V, that path points to the K20 Engine Control Module.
Signal circuit simulation
If the 5 V reference is between 4.8 and 5.2 V, check the scan data with the sensor connector disconnected. Fuel Rail Pressure Sensor 1 should be greater than 249 MPa (36,114 PSI), and Fuel Rail Pressure Sensor 2 should be less than 1 MPa (145 PSI). If those values are not present, turn the vehicle off, disconnect the appropriate K20 Engine Control Module connector, and test both signal circuits at the component harness to ground for infinite resistance. If either signal circuit is less than infinite resistance to ground, repair the short to ground. If the signal circuits show infinite resistance, that path points to the K20 Engine Control Module. If the disconnected scan values are correct, install a 3 A fused jumper wire from Signal circuit terminal 4 to ground and from Signal circuit terminal 2 to ground. Now Fuel Rail Pressure Sensor 1 should be less than 1 MPa (145 PSI), and Fuel Rail Pressure Sensor 2 should be greater than 249 MPa (36,114 PSI). If those values are not present, turn the vehicle off, remove the jumper wires, disconnect the appropriate K20 connector, turn the ignition on in Service Mode, and test both signal circuits at the component harness to ground. They should be less than 1 V. If either one is 1 V or greater, repair the short to voltage. If both are less than 1 V, turn the vehicle off and test each signal circuit from the component harness to the control module harness. Each one should be less than 2 Ω. If either one is 2 Ω or greater, repair the open or high resistance. If both are less than 2 Ω, that path points to the K20 Engine Control Module. If the specified values are present after the jumper-wire check, test or replace the B47B Fuel Rail Pressure Sensor.
Repair verification and takeaway
After the repair is complete, verify the repair and confirm P0191 stays gone. The takeaway is simple: do not condemn the sensor just because P0191 is stored. Prove the two pressure readings, prove the reference voltage, prove the low reference, and prove both signal circuits before calling the sensor or module. For more diagnostic training, visit stepdiagnostics.com.
Final check
P0191 is often diagnosed by comparing the pressure sensor signals and then proving the reference, low reference, and signal circuits before making a final call.
For more guided automotive diagnostics, visit STEP Diagnostics.
