Hello,

I have a Trane XE80 (14 years old) that periodically shuts the gas off but the blower continues to blow trying to heat the house up. If I shut the furnace off, allow it to restart, everything will work fine for a few hours or days. So the occurence is random.

Some background info. About a month ago we had a "shyster" repair tech come and diagnose our furnace for what appeared to be a bad blower fan motor. He said it was bad and that the heat exchanger was cracked, but told us if he fixed it ($1193.00 motor only) we should be ok for while. I got rid of him, and got another tech. It ended up only being a bad capacitor ($12.00), however the motor was squeaky and we determined that the motor shaft was getting loose. We replaced it also ($170.00). So, I am concerned if the first repair tech sabotaged the furnace.

My only other thought is the temperature sensor in the flew area is getting to hot and is shutting the gas off, but I should see the gas turn back on once it cools down and the furnace doesn't appear to be doing this.

Any thoughts?

Thanks!

When the furnace shuts down, look for a diagnostic light on the control board. Do not open the cabinet up to look for it, it will be seen thru a small viewing port in the lower section. Count the number of blinks and record. Now remove the door panel and on the sticker found on the back, there will be a diagnostic chart. Let us know what you get for blinks.

Further diagnoses: OK I have removed the Thermostat from the equation and direclty hot wired the R (red) W (white) contacts/wire. So I now know the Thermostat is not turning the gas off. So the furnace is in permanant on mode. Basically the furnace is on for about 5 minutes, gas shuts off for 2 minutes then back on. It keeps repeating this cycle.

Also, when the gas shuts off (which appears the furnace mey be getting to hot) I get a 4 blinking light pattern which = Thermal Protection Device is open. Otherwise it is in a constant blinking mode which = Normal Operation pulling heat.

I am only guessing but what do think of it being the bi-metal (hi-lo) switch?

Is the room air blower turning on? Have you checked your air filter? How hot is the supply plenum getting (super hot, warm to the touch)? Are all registers open and unrestricted?

Sorry to ask so many questions but there is a reason.

After more diagnosing, I removed the Fan/Limit Temperature Switch from the equation, by hot wiring the to wires together. The furnace ran perfectly. I conitnued testing the switch with a Ohm meter and could see that the switch was opening up after five minutes of operation.

This helped me out.

Furnace is shutting off early and still blowing cool air.? - Yahoo! Answers (http://answers.yahoo.com/question/?qid=20080117075539AAg5d99)