I replaced the drum belt to eliminate the squealing. I replaced Thermal Fuse, the igniter, the flame sensor, cycle thermostat, and the valve coils. I did not bother to test each part to see which one was failed. I figured since I was going to have the entire thing apart anyway, I would just replace everything. At less than $100 bucks it was well worth it. Your parts are reasonable and your how to videos are so easy to follow.