Either your sensor wire is unplugged at the sender, you have a break in that wire (it's blue and plugs into the number eight spot on the white connector that plugs into the back of the cluster), or you have a break in the plastic circuit for the sender current.
The other possibilities are the gauge is bad (not likely) or the sender is bad.
To determine what the problem is, unplug the blue wire at the sender. With the key on, the gauge should read over 80psi. Then ground that wire. The gauge should read well under zero. If you get those readings, your wiring is OK and the sender is probably bad. If you don't get those readings, either your wiring is bad or your gauge is bad. You'll have to pull the cluster. I wrote a paper on how to bench test the gauge. But most likely the wire is OK as the higher probable problem is the plastic circuit on the back of the cluster.
If you send me your email address I'll send you the document.
