Trane XL80 TUD120R954H0 sometimes ignores thermostat

[DamnDummy]

Hi appliantologists,

I have the Trane XL80 with a Trane branded XT302A thermostat. Once in a while, the thermostat kicks on its System light, as it should do when the temperature drops below the desired level, but the XL80 ignores it and does not fire up.

If I go to the unit and power down for a few seconds and then power it up, the furnace will behave as expected and fire up, heating the house.

Any ideas as to what is going on when this happens?

[jumptrout]

You will have to check the low voltage wires at the unit when the light comes on to call for heat.

If the thermostat is sending the signal the board is ignoring it.

If it does not send the signal the thermostat is lying or miswired..

[Shootist]

It could also be that the furnace is locking out over an error. When you power-cycle as you mentioned the board will reset. The next time it doesn't come on see if you locate the LED diagnostic light and check for a blink code. Many Trane furnaces have a little plastic window you can use to view the LED light. If you have an older model without the window you can pop the blower door off and tape the door switch down so you can remove the door to check the LED without killing the power.