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One of the pressure sensors in a gaseous fuel system has failed or drifted.

A gas system measures pressure in more places than a petrol one, because pressure is what keeps the fuel in the right phase. There is tank pressure, the pressure arriving at the regulator, the pressure in the rail, and on liquid-injection systems the hydraulic pressure that stops the fuel boiling before it reaches the injector.

Which sensor is named tells you where to look, and the differences between them are the diagnosis. A healthy tank pressure with a low regulator inlet pressure means a restriction between the two — the line, the shutoff valve, or most often a filter nobody has changed since the conversion was fitted. A pump making pressure into a system that is not holding it means a leak rather than a pump.

A drifted sensor mis-fuels the engine rather than mis-reporting a gauge. Reading high, the module under-delivers and the engine runs lean, which on gas means high combustion temperatures and, over time, damaged valve seats. That is the expensive failure mode of a cheap part, and it is why a lean-running conversion should be diagnosed rather than tuned around.

These sensors are frequently combined with a temperature sensor in one housing, so replacing the assembly often settles both.