P0191: P0191 Diagnostic Guide

What this code means

P0191 may indicate that the engine controller is seeing fuel rail pressure sensor information that does not agree with expected fuel pressure behavior.

What the vehicle may do

The check engine light may be on.
The vehicle may have fuel delivery or drivability symptoms.
The condition can be intermittent if it only appears under certain operating conditions.

Possible fault areas

The fuel pressure and temperature sensor may be involved.
The sensor circuits may be involved.
Connector condition or wiring integrity may be involved.
The engine control module side of the circuit may be involved in some cases.

Diagnostic path

Opening and code meaning

On this 2014 through 2018 GMC Sierra 1500 with the 5.3 gas engine, P0191 is a fuel rail pressure sensor performance code. In plain terms, the engine controller may be seeing fuel rail pressure information that does not line up the way it expects. The truck may turn on the check engine light, and depending on what is actually happening, it can have fuel delivery or drivability symptoms. Broadly, this may involve the fuel pressure and temperature sensor, its low reference, 5 V reference, serial data circuit, connector condition, or in some cases the engine control module side of the circuit. For P0191, the fault logic is based on the difference between Fuel Rail Pressure Sensor 1 and Fuel Rail Pressure Sensor 2 being greater than a predefined value for greater than 2.6 s. As a scan data sanity check, with the engine idling, the normal range is 3.9 to 4.2 MPa (565 to 609 PSI). A listed fault value is 655.4 MPa (95058 PSI), so if you see that kind of reading, treat it as a signal or data clue and keep following the diagnostic path.

Start with verification

Before getting into pinpoint tests, start with the basic system checks and follow a structured diagnostic approach. If other codes are present, check what they mean first instead of chasing this one in isolation. Turn the ignition on with the vehicle in service mode, then first make sure the 5 V reference code for this diagnostic path is not set. If that reference code is set, handle that reference circuit problem first. If it is not set, check whether P0191 or the same fuel pressure sensor code group is currently set. If P0191 is set in that group, the diagnostic path calls for replacing the B310 Fuel Pressure/Temperature Sensor. If the code pattern instead sends you into circuit testing, continue with the electrical checks. If none of the codes are current, reproduce the operating conditions for the code, or reproduce the captured conditions from the stored data. Then verify the code does not reset. If it stays gone, the condition is not present right now. If it resets, move into circuit testing.

Low reference circuit test

For circuit testing, shut the ignition and all vehicle systems off, then disconnect the B310 Fuel Pressure/Temperature Sensor connector. Keep in mind it may take up to 2 min for all vehicle systems to power down before a ground or low reference continuity test is accurate. Start at the low reference circuit. Test between low reference circuit terminal 1 and ground. You should have less than 10 Ω. If it is 10 Ω or greater, disconnect connector X2 at the K20 Engine Control Module and check the low reference circuit end to end, from terminal 1 at the component harness to the matching control module harness end. That circuit should be less than 2 Ω. If it is 2 Ω or greater, repair the open or high resistance in the circuit. If it is less than 2 Ω and the low reference path checks good, the diagnostic path calls for replacing the K20 Engine Control Module.

5 V reference circuit test

If the low reference to ground test is less than 10 Ω, turn the ignition on with the vehicle in service mode. Now test between 5 V reference circuit terminal 3 and low reference circuit terminal 1. The expected reading is 4.8 to 5.2 V. If it is less than 4.8 V, turn the ignition and vehicle off, disconnect connector X2 at the K20 Engine Control Module, and test terminal 3 at the component harness to ground for infinite resistance. If resistance is less than infinite, repair the short to ground on the 5 V reference circuit. If it is infinite, check that same 5 V reference circuit end to end to the control module harness. It 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, replace the K20 Engine Control Module. If the 5 V reference reading is greater than 5.2 V, turn the ignition and vehicle off, disconnect X2 at the engine control module, turn the ignition back on in service mode, and test terminal 3 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, replace the K20 Engine Control Module.

Serial data circuit test

If the 5 V reference is in range at 4.8 to 5.2 V, stay at the sensor connector and test the serial data circuit. Measure between serial data circuit terminal 2 and low reference circuit terminal 1. The expected reading is also 4.8 to 5.2 V. If it is less than 4.8 V, turn the ignition and vehicle off, disconnect connector X1 at the K20 Engine Control Module, and test serial data terminal 2 at the component harness to ground for infinite resistance. If resistance is less than infinite, repair the short to ground. If resistance is infinite, test the serial data circuit end to end 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, replace the K20 Engine Control Module. If the serial data circuit is greater than 5.2 V, turn the ignition and vehicle off, disconnect X1 at the engine control module, turn the ignition back on in service mode, and test serial data terminal 2 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, replace the K20 Engine Control Module.

Sensor decision and repair verification

If the serial data circuit is also in range at 4.8 to 5.2 V, the diagnostic path ends at the B310 Fuel Pressure/Temperature Sensor: test or replace that component. After any repair, keep verification separate from the circuit testing. Verify the repair and confirm the code stays gone under the proper operating conditions. The takeaway is simple: do not guess at P0191. Confirm the reference code status first, prove the low reference, 5 V reference, and serial data circuits, and only then condemn the sensor or module when the test path supports it. For more diagnostic training, visit stepdiagnostics.com.