What you'll notice first
A furnace losing its grip usually tells you before it quits outright. Short cycling, where it fires up and shuts off again within a few minutes, points to a dirty flame sensor or a cracked heat exchanger. A burner that won't light, or lights and then drops out, usually means the igniter is weak or the pressure switch isn't satisfied. Cold air out of the vents while the blower still runs often traces back to the gas valve. A furnace that's noisier than usual, with banging or screeching from the blower compartment, is telling you a motor bearing is wearing out.








