P0191

DTC P0191

Circuit Description

The fuel rail pressure (FRP) sensor is a pressure sensor. The fuel injector control module (FICM) supplies about 5 volts on the FRP sensor reference voltage circuit. The FICM also supplies a ground circuit and a signal circuit to the FRP sensor. When the fuel rail pressure is normal, the FRP signal voltage rises to near 2.5 volts. If the fuel rail pressure increases, the FRP signal voltage increases. The FICM monitors the FRP sensor voltage and communicates the data to the PCM by a dedicated pulse width modulated (PWM) circuit.

This DTC sets when the FRP sensor signal voltage is outside of the normal operating range of the sensor.

Conditions for Running the DTC

- The CNG fuel tank pressure is more than 5,171 kPa (750 psi).
- The ECT is more than 20°C (68°F).
- The engine speed is more than 600 RPM.
- The mass airflow (MAF) is less than 175 g/s.

Conditions for Setting the DTC

- The FRP sensor is less than 827 kPa (120 psi), or more than 1517 kPa (220 psi).
- The condition exists for at least 5 seconds.

Action Taken When the DTC Sets

- The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
- The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records.

Conditions for Clearing the MIL/DTC

- The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
- A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
- A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
- Clear the MIL and the DTC with a scan tool.

Test Description

The numbers below refer to the step numbers on the diagnostic table.

2. This step determines if the condition is present.
4. This step tests the signal circuit, and the low reference circuit of the FRP sensor. The scan tool should display 0 volts when the circuits are jumpered together.
5. This step determines if the signal circuit of the FRP sensor is the cause of the condition. The scan tool should display 0 volts when the signal circuit is jumpered to a good ground.
6. This step determines if the 5-volt reference circuit is the cause of the condition. The DMM should display 5.0 volts across the test lamp.
7. This step verifies that a mechanical fuel system condition is not the cause of the concern. A fuel pressure regulator condition may cause this DTC to set.