1997 Ranger XLT. The gauge was working fine until recently, it just stays on H. It hasn't been full on driven since like October because i owned it then got a new car and gave the truck to my grandpa as a beater. He would start it and let it run more than once a week usually to keep it fresh. It still drives since he drove it today but the gauge just stays on H. Still has all coolant in, no leaks, and runs fine. Guessing its just the gauge not workin properly, any help?

 

Its possible the tstat is stuck closed.

Does it go to the H as soon as the truck starts or does it climb slowly over say 15 minutes?