Well my Thermostat just took a crap on me and threw a "Service Engine Soon" light. A thermostat is either closed or open. I think when it's open you get not so warm heat, as I was getting.
Replacing the Thermostat is not that bad. PFYC has a nice writeup on how to do it. I did it in about 15-25 minutes?.. and it was my first time.