What does it mean?
This code is stored when the voltage on the gaseous fuel rail temperature sensor circuit drops below the minimum valid threshold. In the common thermistor arrangement the module feeds the sensor through an internal pull-up and the sensor pulls the line down as it heats, so a signal near zero volts corresponds to a temperature far higher than the gas rail can ever reach. The module compares the value against that fixed floor, waits for it to persist, then discards it and applies a substitute value for density correction. A short to ground in the signal circuit or an internally shorted thermistor produces exactly this behaviour. Because the substituted temperature is not accurate, the engine can run noticeably worse on gas even though the sensor circuit is the only thing at fault.
Symptoms
- MIL (check engine lamp) on with a gaseous fuel fault stored
- Rail temperature displayed at the very top of the scale on a scan tool even on a cold engine
- Changeover to gas happening too early on a cold start, causing rough running or a stall
- Poor running, hesitation and higher gas consumption once on gas
- Gas fuel trims biased in one direction
- Repeated automatic changeover back to gasoline
Likely causes
- Signal wire shorted to ground, often where the loom rubs a bracket or the engine
- Melted or heat-damaged insulation near the exhaust allowing the signal to touch ground
- Thermistor shorted internally, showing almost no resistance
- Water or corrosion in the sensor connector bridging the signal pin to ground
- Signal wire pinched during a previous repair to the gas system
- Incorrect sensor fitted with a much lower resistance curve
- Control module input circuit failure, the least likely cause
How to fix it
- Read the live rail temperature on a cold engine; a value at maximum with a cold vaporiser proves an electrical fault
- Unplug the sensor and observe the scan tool. A swing to minimum temperature shows the harness is intact and the sensor is shorted
- If the value stays at maximum with the sensor unplugged, the signal wire is grounded and must be traced
- Measure the sensor resistance and compare it with the specification for the ambient temperature
- Inspect the harness for melted or chafed insulation near the exhaust, vaporiser and engine brackets
- Repair the shorted conductor, re-route or shield it away from heat, then clear the code and verify the reading behaves normally through a warm up
- Replace the rail temperature sensor when its resistance is far below specification and the wiring measures good