I wonder if the harness is faulty or the sensor and or gauge. First consult the wiring diagram.. I check ground on the harness first. Are there other electrical problems, possibly related to the same electrical issue? As for the pressure sensor, check for voltage, or, if you remove the wire feeding the sensor and ground it, with someone watching the gauge for movement. If you see movement at the gauge, the fault is the sensor. If no movement run a temporary line and bypass the harness.
If this is successful, make plans to change that temporary title to permanent. Needless to say, harness changing is daunting and unneccesary.