Sending unit is bad, or the unit is no longer grounded.
Most gauges work on resistance, 0 resistance being empty and 90 or so being full. if no ground is present then the circuit won't be complete, on the other hand, if the wire is shorted the gauge should show full.
the fuel guage in the tank is faulty the tank has to be dropped and it replaced either that or is simply a broken wire to the tank
bad sending unit
Yes. The fuel gauge in most vehicles will settle at 'empty' with the ignition off.
chk the ground wire on the harness to the fuel pump...........
Yes, key on, engine off, the fuel gauge should work.
That is the way it is designed. When the key is turned on then it reads correctly.
what kind of car? IF its fuel injected the key turns off the computer, the fuel pump, and the ignition coil
fuel pump is in fuel tank.if pump runs with key off replace fuel pump relay
Fuel pump is locked up. Replace it.
Fuel pressure (ignition key turned to on, engine not running): 50 to 60 psi
proably a bad sensor
get a circuit tester and start looking with the key on