What does it mean?
This fault is the opposite of a low circuit complaint on the same rail temperature sensor. Because the module pulls the signal line up through an internal resistor, an open circuit in the signal wire, the ground path or the thermistor itself leaves the input close to reference voltage, which the strategy reads as an impossibly cold gas rail.
The module compares that value with intake air and coolant temperature and with the ends of the thermistor curve, and stores the code once the signal exceeds the upper threshold for the required time. A false cold reading is significant because many gaseous fuel systems will not change over from gasoline until the rail is warm, so the fault often presents as a vehicle that simply refuses to run on gas. Broken connections are far more common here than a genuine low temperature.
Symptoms
- MIL (check engine lamp) on with a gaseous fuel system fault stored
- Rail temperature reading at the very bottom of the scale even after a long drive
- The system never changes over to gas, or takes far longer than normal to do so
- Rich running and poor economy if the module does run on gas with a false cold value
- Rough idle and hesitation on gas
- Reduced power on dedicated gas vehicles
Likely causes
- Open circuit in the sensor signal wire or its ground return
- Connector unplugged, unlatched or with a backed-out terminal
- Thermistor failed open internally
- Corroded terminals in a connector that has been exposed to engine bay moisture and heat
- Signal wire shorted to a reference or battery voltage from an adjacent circuit
- Harness broken inside the insulation after repeated heat cycling
- Control module input circuit failure, the least likely cause
How to fix it
- Read live rail temperature and compare it with coolant and intake air temperature on a warm engine; a value far below both confirms the complaint
- With the ignition on and the sensor unplugged, jumper the signal pin to ground at the harness connector. A reading that swings to maximum temperature proves the wiring and module are good
- If the value stays at minimum with the pin grounded, trace the signal and ground wires for an open circuit
- Inspect the connector for spread, corroded or backed-out terminals and a broken lock
- Measure the sensor resistance; an infinite reading confirms an open thermistor
- Repair the open wire, terminal or ground and protect the repair from heat, then clear the code and verify the reading climbs during warm up
- Replace the rail temperature sensor when the harness measures good and the element reads open