the temp gauge on my 2008 sonata is going crazy. A couple of days ago i noticed the temp gauge was all the to the red line. There were no signs yet of overheating so i headed home. This was after midnight when i got off work within 5 mile of work. Then the gauge dropped from hot to cold in one motion. It stayed there for about a mile or two then shot back up to hot again. When I got home there was no steam and the hood was cold. It didn't happen again for a couple days and then the same thing, hot to cold and then normal. I took it to my mechanic. He replaced radiator thermostat to no avail. He put it on diagnostic machine and said engine not running hot but thermostat gauge showing all the way to hot.

 

Sounds like a bad guage or sending unit then.

 

You may have a voltage drop on the negative battery cable. This will make temp gauge go haywire. Check the ground wire from plenum to body. Clean / sand contact points. you can also run a redundant ground wire to help the problem