Gauge or sending unit?

[JohnB]

If you ground the sender wire on the back of the gauge to the negative batttery, it should peg to full. Gauges rarely go, although mine went this year. The most common cause of this is a bad ground from the battery to the gas tank. I would check that first. If the gauge checks out, your going to have to get at the sender to check the wires. Check that you have resistance between the ground plate on the top of the sensor where the ground tab is, and the negative on the battery. Finally, check the sender wire, they can corrode and go bad too. Check both the wire and the connection. If you ground the sender wire off the tank, the gauge should peg full. All this needs to be done with the key on. Worst case, pull the sensor, and put a meter between the ground and the sensor wire, and when you move the float up and down, you should see the resistance change.