Does the car crank but not start at all after you fill up? Have you tried starting it without the gas cap? Sounds like you're getting some vapor lock. I'm assuming the car starts fine all the other times you use it, but only has a problem when you open the gas cap and fill it up?
Check the wiring from the signal light back into the wire loom. One of the wires might be chaffing on a sharp metal edge. Also, make sure the light socket is free of rust/debris.
Don't have much info for you on the temp sensor problem other than seeing if it'd be affordable to just try replacing it and then see how that works. Otherwise, to test it you'd need resistance readings at certain temps, subject it to those temps and then read the resistance to see if it's reading correctly.