What does it mean?
This code reports that the signal from the gaseous fuel rail pressure sensor has fallen below the lowest voltage the circuit can legitimately produce. On a healthy system the sensor is powered by a 5-volt reference and idles at a defined offset voltage even when the rail is completely empty, so a reading close to zero is an electrical failure and not simply no gas pressure.
The gaseous fuel control module compares the signal against a fixed low threshold, waits for it to persist across a calibrated period, then stores the code and stops trusting the value. Since the module needs rail pressure to compute injector opening time, it will normally apply a default value, restrict gas operation, or command a changeover to gasoline.
Diagnosis should separate a genuine short or open reference from an actual loss of rail pressure, because the two feel very similar to the driver.
Symptoms
- MIL (check engine lamp) on and the vehicle running on gasoline instead of gas
- Refusal to change over to gas, or an immediate return to gasoline after the changeover
- Rough running, misfire or a stall on gas where the module still attempts gas operation
- Reduced power and poor throttle response
- Gas fuel trim values that are clearly out of range on a scan tool
- Hard starting on gas in cold conditions
Likely causes
- Signal wire shorted to ground, often where the loom is chafed against a bracket or the engine
- Loss of the 5-volt reference to the sensor, which forces the output to zero
- Water or corrosion in the sensor connector bridging the signal pin to ground
- Internally failed rail pressure sensor with a shorted output
- Damaged or melted harness insulation where it runs near the exhaust
- Faulty ground routing causing an unusually low offset voltage
- Control module reference or input circuit failure, which is uncommon
How to fix it
- Read the live rail pressure with the system pressurised; a reading at or near zero while the rail is full of gas points at the circuit
- Unplug the sensor and check whether the scan tool value jumps to maximum. If it stays at zero, the signal wire is grounded somewhere
- With the sensor unplugged and the ignition on, measure the 5-volt reference and the ground at the harness connector
- Test the signal wire for a short to ground with the harness disconnected from the module, working along the loom near heat sources and brackets
- Repair the chafed, melted or shorted conductor and re-route or protect the harness so the damage cannot recur
- Confirm actual rail pressure with a mechanical gauge if there is any doubt that gas is reaching the rail
- Replace the rail pressure sensor once the reference, ground and signal wiring have all been proven good