What does it mean?
On bi-fuel and dedicated gaseous fuel vehicles, tank A pressure sensor reports the pressure of the stored CNG or LPG to the fuel system control module. Together with tank temperature, that pressure is how the module calculates the remaining mass of gas, detects that a refill has taken place, and decides when the engine must be handed back to gasoline. The sensor is normally a three-wire unit fed by a 5-volt reference, and its output stays well above zero volts even with a nearly empty tank.
This code sets when the module sees the signal pinned at or under the bottom of the legal window, typically below roughly 0.2 to 0.5 volts, for a calibrated period. That is a level the sensor cannot produce electrically no matter how little fuel is left, so the value is discarded as invalid rather than treated as a low tank.
Symptoms
- MIL (check engine lamp) illuminated with the fault stored in the gaseous fuel controller
- Gas fuel gauge reads empty or does not move even immediately after a refill
- Vehicle refuses to change over to gas and stays on gasoline
- Unexpected switch back to gasoline while driving on gas
- On dedicated gas vehicles, extended cranking or a no-start because gas delivery is not authorized
- No noticeable change in engine running quality once gasoline operation takes over
Likely causes
- Signal wire shorted to ground between the tank sensor and the control module
- Missing or shorted 5-volt reference feed, which drags the signal line to zero
- Water intrusion and corrosion inside the tank sensor connector, common because the sensor sits under the vehicle
- Internally failed pressure sensor with a collapsed output stage
- Chafed or crushed harness where the loom crosses the tank straps or an underbody bracket
- High resistance in the sensor ground, offsetting the signal below the valid range
- Rarely, a failed reference supply or input circuit inside the fuel system control module
How to fix it
- Record freeze frame data and read the live tank pressure value with a scan tool; a reading stuck at the very bottom of the scale confirms an electrical fault rather than an empty tank
- Unplug the tank sensor connector and inspect for green corrosion, spread terminals and trapped water before any testing
- With the connector off and the ignition on, measure the 5-volt reference and the ground at the harness side; a missing reference means the problem is upstream of the sensor
- Watch the scan tool value with the connector disconnected. If the signal climbs to full scale, suspect the sensor; if it stays at zero, the signal wire is shorted to ground
- Check the signal wire for continuity and for a short to ground with the harness free at both ends, flexing the loom along its underbody run
- Repair the damaged conductor, terminal or ground point, then clear the code and confirm the pressure value tracks the real tank state after refuelling
- Replace the tank pressure sensor only once the wiring, reference and ground have all measured good