What does it mean?
The alternative fuel rail temperature sensor measures the temperature of the gas as it arrives at the injectors, downstream of the vaporiser or regulator. That temperature is essential because gas density falls as it warms, so the module must combine rail temperature with rail pressure to know the actual mass of fuel each injector pulse will deliver.
The sensor also protects the system by telling the module when the gas is still too cold to be injected safely after a cold start, which is why many installations delay the changeover from gasoline until it warms up. This general circuit code is set when the module cannot obtain a valid reading from the sensor, either because the signal is out of range or because the circuit is not responding at all. Gas metering then falls back on a default value and running quality on gas suffers.
Symptoms
- MIL (check engine lamp) on with a gaseous fuel system fault stored
- Changeover from gasoline to gas delayed, refused or repeatedly aborted
- Rough running, hesitation or misfire on gas, worst when the engine is cold
- Higher than normal gas consumption
- Gas fuel trims drifting well away from centre
- Reduced power on dedicated gas vehicles
Likely causes
- Open or shorted wiring between the rail temperature sensor and the gaseous fuel control module
- Corroded or loose connector at the sensor, which sits in a hot engine bay location
- Failed thermistor, either open circuit or shorted internally
- Heat damaged harness where the loom passes close to the exhaust or the vaporiser
- Poor sensor ground or a corroded ground point
- Sensor loose in its port, or a combined pressure and temperature sensor that has partly failed
- Control module input circuit failure, the least likely cause
How to fix it
- Read the live rail temperature after an overnight soak and compare it with intake air and coolant temperature; on a cold engine they should be close
- Inspect the sensor and connector for heat damage, melted insulation, corrosion and loose terminals
- Measure the sensor resistance and compare it with the specification for the current temperature; an open or a dead short identifies a failed thermistor
- With the sensor unplugged and the ignition on, confirm the reference voltage and a good ground at the harness connector
- Test the signal wire for continuity, short to ground and short to voltage back to the module
- Repair the damaged wiring, re-route or shield the loom away from heat, then clear the code and confirm the reading rises normally as the engine warms
- Replace the rail temperature sensor when the wiring is good and the resistance is out of specification