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This code is strictly about the incoming signal from the FRP (fuel rail pressure) sensor falling below the calibrated limit for a calibrated amount of time. This code can either be a mechanical or electrical problem, which largely depends on the manufacturer, fuel system or fuel type of the vehicle.
This code is a generic trouble code, meaning it applies to all vehicles equipped with OBD-II equipment, most especially to fuel injected engines, both gas and diesel vehicles made in 2000 up to present, and usually to manufacturers such as Ford, GMC, Volvo or Volkswagen. Specifications on the definition, troubleshooting and repair of course, vary from one make/model to another.
The PCM needs to know the fuel pressure at all times so it can control the fuel injectors effectively and deliver the right amount of fuel to the engine’s combustion chamber and allow it to have the maximum power and fuel economy, all while maintaining the lowest possible emission on the tail pipe.
Error Code P0191 indicates there’s a problem in the Fuel Rail Pressure Sensor and/or its circuit, as the PCM read erratic readings from it, suggesting abnormal fuel pressure condition.
As with other error codes, this trouble code illuminates the check engine light. Other symptoms include:
- Engine hesitation or stalling
- Engine cranks but will not start
- Engine dies or stalls at stop signs
- Excessive fuel consumption
- Either Rich fuel trim (P0172) or lean fuel trim (P0171) code may be triggered
- Tail pipe emission has unusual smell
- Vehicle fails emission test
Some of the most common causes of this error code include:
- Low or high fuel pressure
- Low or no fuel
- Defective FRP sensor, circuit or connector
- Defective fuel pressure regulator
- Vacuum leaks
- Excessive circuit resistance
How to Check
As with most OBD-II codes, mechanics start their diagnosis for this error code using OBD-II scanner; they retrieve freeze frame data to look for the conditions of the vehicle when the code was set.
Then, they will take the vehicle for a test drive and look through the live data stream whether the reading for the fuel pressure is normal.
A fuel pressure tester will be used to determine whether there’s a problem in the fuel pressure or not.
An oscilloscope will be used to test the connectors and wirings of the fuel rail pressure sensor for vehicles with normal fuel pressure. This is to confirm the sensor’s quality and performance.
If the actual fuel pressure is normal and the circuitry of the sensor is unimpaired, then that suggests a problem in the sensor.
How to Fix
Repairs for this error code are pretty simple and straightforward, and should be clear following a thorough diagnosis. This includes:
- Replacement of defective fuel pump
- Replacement of fuel pressure regulator
- Repair or replacement of corroded connectors to the fuel pressure sensor
- Repair or replacement of broken, frayed or shorted wires leading to the fuel pressure sensor
- Replacement of fuel pressure sensor
- Replacement of engine vacuum leaks
Error Code P0191 is considered a serious trouble code, as it causes drivability issues such as stalling or hesitates while driving, which could pose safety risks. Also, it can cause excessive fuel consumption.
One of the most common mistakes people do when dealing with this trouble code is overlooking other components and proceeds on repairing or replacing the fuel trail pressure sensor.
Also, chaffed and insufficiently connected wiring, faulty fuel pressure regulator and fuel pump are often neglected when diagnosing and conducting repairs for this error code.