The first is that the sender is bad, the second is the guage is bad. Two other possible solutions are a bad instrument cluster voltage regulator, and finally a problem in the wiring going to the sender.
There is a pretty easy test. Turn the key to on, and remove the little wire on top of the sender. Ground this wire to the engine block. The needle on the guage should swing way up. If it does, you have a bad sender. This would be the most common scenario.
If the guage still doesn't work, measure for voltage between ground and the wire to the sender (remove the wire from ground before doing this). If I recall , there shoudl be approx 5VDC present with the wire disconnected. If you have no voltage, the insturment cluster regulator is bad. I had this happen to me just this past fall.
Finally, double check the wiring. I do not know how to test the guage, but I would say that if you have gotten this far with no luck, the next thing to do would be to swap out the guage itself.
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