What does it mean?
The alternative fuel rail pressure sensor sits downstream of the pressure regulator or vaporiser and measures the gas pressure actually delivered to the gaseous fuel injectors. Because CNG and LPG are compressible, the module cannot calculate an injector opening time from load alone; it needs the real rail pressure, combined with rail temperature, to work out fuel density and deliver the correct mass of gas.
This general circuit code is stored when the module cannot obtain a usable signal from that sensor at all, either because the electrical circuit is out of range or because no plausible response is present. It is a circuit-level complaint rather than a judgement about the pressure value itself, so the module usually applies a default rail pressure or drops the engine back to gasoline.
Fuel metering accuracy suffers immediately, which is why the engine often runs poorly on gas until the circuit is repaired.
Symptoms
- MIL (check engine lamp) on, with the gaseous fuel system fault stored
- Engine will not switch to gas, or switches back to gasoline shortly afterwards
- Rough running, hesitation or misfire while operating on gas
- Noticeably higher gas consumption and poor throttle response
- Reduced power or limp mode on dedicated gas vehicles
- Hard starting on gas, especially when the engine is cold
Likely causes
- Open, shorted or chafed wiring between the rail pressure sensor and the gaseous fuel control module
- Corroded, wet or loose connector at the sensor, which lives in a hot, vibrating area near the rail
- Failed rail pressure sensor element or internal electronics
- Missing 5-volt reference or a shared reference pulled down by another failed sensor on the same circuit
- Poor sensor ground or a corroded ground stud in the engine bay
- Heat damage to the harness where it passes close to the exhaust or turbocharger
- Control module input or reference supply failure, the least likely cause
How to fix it
- Read live data and compare the rail pressure reading with the value expected after the regulator; a flat, missing or clearly nonsensical value confirms the complaint
- Inspect the sensor and its connector for heat damage, oily residue, corrosion and loose terminals
- With the ignition on, verify the 5-volt reference and ground at the sensor connector; a dead reference frequently affects several sensors at once, so scan for companion codes
- Check the signal wire end to end for continuity, short to ground and short to battery with the module unplugged
- Compare the sensor output against a mechanical gauge tapped into the rail where the system allows, to confirm whether the sensor is reporting truthfully
- Repair the damaged wiring, terminal or ground, then clear the code and verify the rail pressure signal follows the regulator during a run on gas
- Replace the rail pressure sensor only after supply, ground and signal wiring have all tested good