My Trane XL80 is experiencing an intermittent problem. Sometimes it runs fine. Other times it will come on, heat the house and then the fan will run, and run, and run while the house gets cold. This is becoming more and more frequent.
Sometimes cycling the power will effectively reset the system and it'll work again. For how long it works again, varies. Sometimes days while other times it runs just once before I have to cycle the power again.
I've checked the TCO and its still has continuity, the heat sensor resistance is lower while hot and higher when cold. I've cleaned it with steel wool to remove any carbon build-up. The 1/2" of water pressure switch also seems to work just fine. The auto-ignitor glows for upwards of a minute before the system shuts it off when the unit is misbehaving. According to the ignitor control box it should only have to run for 6.8 seconds. While watching the sequence, its apparent that the gas valve is not being opened. What could be telling the system to prevent it from opening the gas valve. I've also inspected the circuit board and there is no sign of damage or overheating.
Thx in advance, tocguy