Rail pressure sensors, unless skewed or setting a circuit code, will not cause a no start or low rail pressure. They are a messenger to the ECM /PCM. A rail pressure sensor reports what the rail pressure is. If the rail pressure is low, then it will report low rail pressure, and low rail pressure is normally caused by different problems.
The high pressure pump creates high pressure at low volume. Depending on the system, it is up to the high pressure limit valve, injector connector tubes, fuel pressure control solenoid and injectors to hold leakage and allow the pressure to build. A fuel supply side restriction will also cause low rail pressure. If the connector tubes do not seat or the injectors have too much internal leakage, which returns to the tank, then the pump cannot produce enough pressure for starting.
Dodge / Ram / Cummins – 5.9 L (no start hard start diagnostic) rail pressure KOEO (Key On, Engine Off) should be 0 +/- 500psi. 6.7 L (no start hard start diagnostic) rail pressure sensor .5 volts KOEO. P0191 Fuel Rail Pressure Performance; could be a bad rail pressure sensor or reporting that the rail pressure is too low. If the rail pressure sensor fails, the ECM will substitute a default value.
Duramax (diagnostics) 6.6 LB7 & LLY KOEO (Key On Engine Off) rail pressure sensor should read 145 – 261 psi (1.0-1.8 Mpa) LBZ & LMM Duramax KOEO should be 0 psi. A skewed rail pressure reading will not cause a hard start.
Sprinter & Jeep rail pressure sensor should read as noted below
- 2007-2009 3.0 L Sprinter .45 – .60 volts KOEO (diagnostics)
- 2001-2006 2.7 L Sprinter .47 – .53 volts KOEO (diagnostics)
- 2005-2006 Jeep CRD Liberty .45 – .55 volts KOEO (diagnostics)
- 2007 – 2008 Jeep 3.0 L Cherokee .45 – .60 volts KOEO (diagnostics)
The rail pressure sensor has a 3-wire connection. 5-volt reference, ground and signal wire (pressure voltage signal), which sends the reading to the ECM / PCM.
Except for the Ford 6.7 L Powerstroke, a low rail pressure reading, with no codes, will not cause a no start.
 6.7Ford Powerstroke injectors will not fire until the FRP sensor reports enough pressure.