Temperature gauge
Probably the regulator is bad in the cluster (could be in the gas gauge or a separate can on the back of the cluster) or the dash has lost its ground to the body. If you have a DVM or test light you should see a pulsing 12V to 0V at the blue wire that plugs into the sensor. A good Fluke DVM will not catch the full 12V (only on for 100ms or so) but you will see the meeting fluctuating a volt or two between the wire and ground.
