P0191

Advanced Diagnostics - DTC P0191: Fuel Rail Pressure Sensor Circuit Range/Performance Problem






General Description
The compressed natural gas (CNG) is supplied from the fuel tank, and it is regulated to about 250 kPa (depending on the manifold absolute pressure (MAP) sensor value) by the fuel pressure regulator before it reaches the engine. The powertrain control module (PCM) monitors the fuel pressure value detected by the fuel pressure sensor (PF2A) to maintain the correlation with the MAP sensor value. The PCM uses two test modes: Modes 1 and 2 based on the correlation between the MAP and PF2A shown in the graph to detect abnormal sensor output characteristics.

Mode 1: If the variance of PF2A is within a set range when the MAP varies beyond a certain value, no malfunction is detected, and Mode 2 is started. If the variance of PF2A is out of a set range, the PCM detects a malfunction and stores a DTC.
Mode 2: If the calculated PF2S is out of a set range, the PCM detects a malfunction and stores a DTC.

Monitor Execution, Sequence, Duration, DTC Type, OBD Status






Enable Conditions






Malfunction Threshold
Mode 1: The difference between PF2A values when the MAP is maximum and when the MAP is minimum, is 4 kPa (0.04 kgf/cm2, 0.5 psi) or less.
Mode 2: PF2A is not between 204 - 326 kPa (2.1 - 3.3 kgf/cm2, 30 - 47 psi) for at least 3.0 seconds. (PF2S = PF2A - MAP)

Driving Pattern






1. Start the engine. Hold the engine speed at 3,000 rpm without load (in P or N) until the radiator fan comes on.
2. Drive the vehicle, and gradually accelerate and decelerate for at least 1 second. (Mode 1)
3. Drive at a speed between 35 - 75 mph (56 - 120 km/h) for at least 3 seconds. (Mode 2)

- If you have difficulty duplicating the DTC, retest after turning off electrical components such as the audio system and A/C, and try a different gear position.
- Drive the vehicle in this manner only if the traffic regulations and ambient conditions allow.

Diagnosis Details

Conditions for setting the DTC
When a malfunction is detected during the first drive cycle, a Pending DTC is stored in the PCM memory. If the malfunction returns in the next (second) drive cycle, the MIL comes on and a Confirmed DTC and the freeze data are stored.

Conditions for clearing the DTC
The MIL is cleared if the malfunction does not return in three consecutive trips in which the diagnostic runs. The MIL, the Pending DTC, the Confirmed DTC, and the freeze data can be cleared with the scan tool Clear command or by disconnecting the battery.