My tenants have a Trane XL 80 2 stage furnace. The unit works fine on the first stage. I know after about 10 minutes a second stage kicks in and the furnace blows harder. The second stage is no longer kicking in. I opened door on furnace and vacuumed dirt. Furnace filter has been changed. I cleaned flame sensor for the heck of it. There are 2 pressure switches, One labeled "low" and one labeled "high" I believe one of these might be the problem, but I am not very sure how to troubleshoot the pressure switches. Any help appreciated.

How about starting with a REAL model number. There are many XL80 units out there.

I'm sorry. I didn't know there were different models of XL 80's. My model number is TUD140R960H4

Be happy! Second stage is more costly to run. Wear more blankets around the house... :D

Have you checked the self diagnostic control in the blower area? There is a little peep hole covered with a plactic cover in the blower door. When the unit is running it should give a number of flashes. The legend for those flashes are on the back side of the blower door with the wiring diagram.

Did you turn the thermostat all the way up to make sure the thermostat is calling for the second stage and wait a minute or so to see if it kicks in?

Try these items first.

I just watched furnace for a while. Furnace runs for 10 minutes in stage 1. It then tries to go to stage 2. The flame will go out and I will get 4 blinking lights which indicates thermal protection device open. After a few minutes flame will come back on and stage 1 will repeat. I have 25 volts on both terminals of both limit controls. I get 24 volts on my low pressure switch. I assume I am getting 0 volts on the high pressure switch because furnace isn't in stage 2.

Track down the thermal protection device and replace. That should take care of it. Now on some units there is a very small reset button on the thermal device and it can be reset but many are one time deals.

Here is a information booklet on those style furnaces.

Trane 40in Gas Furnace Operation (99) (http://www.scribd.com/doc/7267240/Trane-40in-Gas-Furnace-Operation-99)

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