What does it mean?
The alternative fuel tank temperature sensor is a thermistor mounted in or on the CNG or LPG tank, and its job is to let the module convert tank pressure into a meaningful quantity of fuel. Gas density changes sharply with temperature, so the same pressure can represent very different amounts of stored fuel on a cold morning and after a long summer drive. The module also uses the temperature to detect refuelling, since filling a tank raises both pressure and temperature together.
This general circuit code is stored when the module cannot obtain a valid reading from that sensor at all, without necessarily deciding whether the signal is too high or too low. The immediate consequences are an unreliable gas gauge and, on many systems, a restriction on gas operation until the reading is trusted again.
Symptoms
- MIL (check engine lamp) on with a gaseous fuel fault stored
- Gas fuel gauge inaccurate, sticking or reading empty with fuel in the tank
- Range estimate that does not match actual consumption
- Refill detection failing to update the gauge after filling
- Changeover to gas refused or delayed, especially in cold weather
- On dedicated gas vehicles, cold start difficulty
Likely causes
- Corroded or water-filled connector at the tank temperature sensor, which sits in an exposed underbody location
- Open or shorted signal wire between the sensor and the gaseous fuel control module
- Failed thermistor element, either open circuit or shorted internally
- Chafed harness where it passes the tank straps, brackets or the underbody floor
- High resistance sensor ground or a corroded ground point
- Rodent damage to the underbody loom
- Control module input circuit failure, the least likely cause
How to fix it
- Read the live tank temperature value and compare it with ambient temperature on a vehicle that has been standing overnight; a wildly wrong or missing value confirms the fault
- Unplug the sensor and inspect for water, corrosion and damaged terminals before making any measurements
- Measure the sensor resistance and check it against the expected value for the current temperature; an open or a dead short identifies a failed thermistor immediately
- With the sensor unplugged and the ignition on, check for reference voltage at the harness connector and a good ground
- Test the signal wire for continuity, short to ground and short to voltage back to the module
- Repair the damaged wiring or connector, seal it properly against water, then clear the code and verify the temperature reading follows ambient conditions
- Replace the tank temperature sensor when the wiring is proven good and the resistance is out of specification