I'm assuming you're talking about the cooling system thermostat? You only replace this when it goes bad. Usually the car will overheat when you have this problem. But that's if it's stuck closed. If it's stuck open you'll find it very hard to heat your vehicle and the temperature will not rise much off of the C.
〉 Answered on Sep 12th, 2012 by Lori Johnson, Owner and Instructor at Ladies Start Your Engines