instrument panel malfunction fuel guage will read full all the time
Someone will have to figure out where the malfunction lies: the sender unit in the tank (most likely - part of the fuel pump assembly usually), the gauge itself, or a connection between. Good luck and please let us know what you find.
When there’s a 1/4 tank of gas, reach under the car and hit the bottom of the gas tank with a rubber mallet, and see if the gas gauge drops.
My truck has the opposite problem, it always reads empty on one of its tanks. On mine, the problem is what is called the “sender” unit, which is just a float that floats on top of the gas inside the tank, and when the gas level changes the float arm moves and this arm position info is conferred on wires from the tank to the instrument panel for display. The way I figured out the sender was the problem is I removed the wires from the float arm (this is outside the tank) and installed some resistors in a test-configuration to the wires instead. One resistor combination should cause the gauge to go to 1/4, another to 1/2, etc. That all worked, which proved the problem is the sender. Your shop could do the same thing. Then at least you’d know if it was the sender or somewhere between the sender & the instrument panel.