For starters, use a paperclip bent into a small hook to pull the little grille out of the dash, just left of the steering column. Inside is a small thermistor bead and further in is the module board with a small computer muffin fan on it. The fan either gets extremely dusty (it runs all the time) or it's frozen - the sleeve bearing dries out. Blow out the tunnel with a can of computer compressed air. Dust and crap will go down and out under the steering column.
If you can't hear the fan whirring, the module comes out - stick your finger in the vent duct grommet in the end of the dash - the one that feeds the window vent in the door. Yank the end off the dash. Straight in there's a small hex head sheet metal screw. Remove it - you'll need a set of spring fingers or a magnet to pull it out the last thread or so. Fish out the module and disconnect it. If the fan can be turned, pull the seal tape off the center of the hub. There will be a bearing and E-clip in there. Put one drop of engine oil on the axle and spin the fan to work in the oil. Replace the seal tape or make a new one. Replace the sensor. Small fingers and magic words.