What does it mean?
This performance code means the gaseous fuel rail temperature reading is electrically valid but does not make sense against the rest of the picture. The module knows the rail temperature must climb after a cold start as coolant heats the vaporiser, must sit within a predictable band once the engine is at operating temperature, and must agree closely with ambient and intake air temperature after a long soak.
When the value stays frozen, warms far too slowly, or is offset from the reference sensors by more than the allowed margin, the module concludes it cannot be used for density correction. The effect is a gas mass calculation that is systematically wrong, which shows up as fuel trim drift and poor running on gas rather than as a dead gauge. Both a lazy sensor and a vaporiser that is not being heated properly can produce this fault.
Symptoms
- MIL (check engine lamp) on with a gas system rationality fault stored
- Extended delay before the system changes over from gasoline to gas
- Rough running or hesitation on gas, particularly during warm up
- Gas fuel trims drifting rich or lean in a consistent direction
- Increased gas consumption without any other obvious fault
- Cabin heater performance also poor when a coolant circulation problem is the root cause
Likely causes
- Drifted or aged thermistor reporting a constant offset
- Insufficient coolant flow through the vaporiser, so the gas genuinely never reaches the expected temperature
- Sensor loose or incorrectly seated in its port, measuring housing temperature instead of gas temperature
- High resistance in the signal or ground circuit shifting the reading without leaving the valid range
- Wrong sensor fitted during a previous repair or conversion, with a different resistance curve
- Engine thermostat stuck open, keeping the whole coolant circuit and the vaporiser too cool
- A reference sensor used in the comparison being faulty in its own right
How to fix it
- Read rail temperature, coolant temperature and intake air temperature together on a cold vehicle; they should agree closely, and the outlier is the suspect
- Warm the engine and watch rail temperature rise; a reading that stays low while coolant is at operating temperature points at the vaporiser circuit, not the sensor
- Check the coolant hoses to and from the vaporiser for flow, blockage, air locks and correct connection
- Measure the sensor resistance at a known temperature against the specification to expose drift or a wrong part
- Voltage drop test the signal and ground wires, since added resistance shifts the reading by several degrees
- Verify the sensor is fully seated and sealed in its port and that the correct part number is fitted
- Replace the rail temperature sensor once the coolant circuit and the wiring have both been proven good