What does it mean?
Here the gaseous fuel rail pressure sensor signal is sitting at or above the highest voltage the module accepts as valid. A functioning sensor never reaches the top of its 5-volt range in normal service, because the regulator holds rail pressure well below the sensor full-scale value, so a reading pinned at the ceiling is treated as a circuit fault. The most common electrical cause is an open in the signal or ground path, which lets the module pull-up drag the input to reference voltage. The gaseous fuel controller stores the code once the condition persists, discards the value and falls back on a default rail pressure or on gasoline operation. A genuinely over-pressurised rail is possible but rare, and it usually appears alongside regulator complaints rather than on its own.
Symptoms
- MIL (check engine lamp) on and gas operation blocked or cut short
- Very rich running, black smoke or a strong gas smell if the module acts on a false high pressure
- Rough idle, misfire or stalling shortly after changing over to gas
- Poor gas economy and abnormal gas fuel trim readings
- Automatic and repeated fallback to gasoline
- Reduced power on dedicated gas vehicles
Likely causes
- Open circuit in the sensor signal wire or an unplugged sensor connector
- Open or high resistance sensor ground, preventing the output from being pulled down
- Signal wire shorted to the 5-volt reference or to battery voltage from a neighbouring circuit
- Internally failed sensor stuck at maximum output
- Corroded or backed-out terminal in the sensor connector
- Failed or stuck gaseous fuel pressure regulator allowing genuinely excessive rail pressure
- Control module input circuit failure, the least likely possibility
How to fix it
- Read live rail pressure with the ignition on and the system unpressurised; a value at full scale with no gas in the rail confirms an electrical fault
- Inspect the sensor connector for pushed-out pins, corrosion, heat damage and a loose lock
- Ignition on, ground the signal pin at the harness connector with the sensor removed. If the scan tool value falls to zero, the harness and module are good and the sensor is faulty
- If the value stays high with the pin grounded, test the signal wire for an open and for a short to the 5-volt reference or battery positive
- Check the sensor ground for continuity and voltage drop back to its termination point
- Where the wiring is proven sound, verify actual rail pressure mechanically before assuming the sensor is wrong, because a stuck regulator can create real over-pressure
- Repair the open or short, or replace the sensor, then clear the code and confirm the reading tracks the regulator during a run on gas