What Does P0191 Code Mean?

OBD-II Code P0191 is defined as a Fuel Rail Pressure Sensor Circuit Range/Performance

The Power Train Control Module or PCM needs to know the fuel pressure at all times. This is so the PCM can effectively control the fuel injectors so they deliver the optimal amount of fuel into the engine's combustion chambers for maximum power and economy while maintaining the lowest tail pipe emissions.

Code P0191

indicates that the signal from the Fuel Rail Pressure Sensor and/or its circuit is sending erratic readings, indicating a fuel pressure condition that is abnormal.

P0191 Symptoms

  • Check Engine Light will illuminate
  • Fuel Consumption may be higher than normal
  • Tail pipe emissions may smell unusual
  • Engine hesitation or stalling
  • Engine may die/stall at stop signs
  • Rich Fuel trim codes (P0172 fuel system to rich) may be triggered
  • Lean Fuel trim codes (P0171 fuel system too lean) may be triggered
  • Vehicle fails emissions test
  • In rare cases, no abnormal symptoms may be noticed

Common Problems That Trigger the P0191 Code

  • Defective Fuel Pressure Sensor 
  • Defective Fuel Pressure Sensor Circuit or connector
  • Damaged/Defective Fuel pressure regulator

Common Misdiagnoses

  • Fuel Rail Pressure Sensor is replaced when the cause is a defective Fuel Pressure Regulator
  • Fuel Rail pressure Sensor is replaced when the cause is a defective fuel pump
  • Fuel Rail Pressure Sensor is replaced when the cause is a poor connection or chafed wiring

P0191 Diagnostic Theory for Shops and Technicians

First thing to do is to gather the Freeze Frame Data in order to analyze the engine's operating conditions when the code was set. Then, test drive the vehicle with a quality data streaming scan tool and watch the fuel pressure PID. Do the fuel pressure readings seem normal? If not, test the actual fuel pressure with a mechanical fuel pressure gauge to see if the sensor and/or its circuit is at fault or, if there is a fuel pressure problem. If the fuel pressure is normal, then test the Fuel Rail Pressure Sensor connector and wiring with a labscope to verify the circuitry is intact. If the circuitry is intact, and the 'real' fuel pressure is normal, then the Sensor is most likely at fault. If the fuel pressure is erratic i.e  too high, then too low etc I would inspect the fuel delivery system in order to determine the cause of the erratic fuel pressure.  I would also perform an amperage draw on the fuel pump to see how much current it is using.

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