If you want to see if the thermostat is bad - toss the old one in a pan
of hot water on your stovetop and see if it opens when it reaches the
desired temperature (use a candy thermometer to test the water temp if
you need to). Normally, exact temps don't matter - the stat will ether
pop open when the water starts getting really hot, or it'll never pop
open if its bad, but sometimes they are just slow (and tus the candy
thermometer is a good tool)