Possible causes

– High fuel pressure
– Low fuel pressure
– Damaged FRP sensor
– Excessive resistance in the circuit
– Low or no fuel
– Wiring harness connector of the fuel pressure sensor
– Fuel Rail Pressure Sensor
– Vacuum leaks
– Fuel

Symptoms

– Engine Light ON (or Service Engine Soon Warning Light)
– Engine will not start

P0191 FORD Description

The Fuel Rail Pressure (FRP) sensor is a diaphragm strain gauge device in which resistance changes with pressure. The electrical resistance of a strain gauge increases as pressure increases, and the resistance decreases as the pressure decreases. The varying resistance affects the voltage drop across the sensor terminals and provides electrical signal to the Powertrain Control Module (PCM) corresponding to pressure.
Strain gauge type sensors are consider passive sensors. A passive sensor is connected to a voltage divider network so that varying the resistance of the passive sensor causes a variation in total current flow.
Voltage that is dropped across a fixed resistor in series with the sensor resistor determines the voltage signal at the PCM. This voltage signal is equal to the reference voltage minus the voltage drop across the fixed resistor.
The FRP sensor measures the pressure of the fuel near the fuel injectors. This signal is used by the PCM to adjust the fuel injector pulse width and meter fuel to each combustion cylinder.

FORD Technical Service Bulletins for P0191 code