Also there's the variable-speed furnace/AC fan. These heavy bastards, with an add-on brain, are very expensive when they go bad. In my case, the brain part was fine, but the fan motor died. They wouldn't sell me just the motor, just the combo for $900US! And, if I install the combo, it voids the warranty. (I did install it, I'm the homeowner not a repairman.)

I was also told if my unit was a Trane, they weren't allowed to sell me the combo! (My unit is a Goodman.) What a rip off!