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My Trane XR80 furnace has an "open temperature limit circuit" problem, per the manual and the four flash sequence of the red diagnostic light. The furnace would fire up and work but the heat would shut off too early and the fan would keep going and end up blowing unheated air, then as time went on for a week or so the furnace would not light at all. It seems to have started intermittent and got worse. I think I have located the part, which looks like a flat piece, maybe two by three inches with two wires attached and it has two screws holding it in place - looks like an easy one to replace. Does this sound like the problem part and any tips on getting a part and doing the fix?
These rarely go bad. The problem is with your air flow not with the switch as the switch is probably doing exactly what it is there for. This switch senses the temperature inside the furnace cabinet and if the heat is not being pushed out and into the house properly it will build up this heat and trip the limit to protect you and your equipment. This is caused by a dirty filter, a dirty evaporator coil (number one reason if the evaporator is over 10 years of age), an improper rpm output or malfunctioning blower motor.
The best way to go about testing the limit is to first find out what the limit cut off temperature is. On the limit it will have something like 140-20 or 160-20. The 140 is the high temperature cut off and then the 20 means that when the blower gets the heat out and temperature back down to 120 it will allow the burners to come back on. Once you know the high temp cut off, insert a temperature probe into the top of the furnace just under the drain pan of the evaporator coil which sits on top of the furnace. Then monitor the temperature of the heat build up in the cabinet when it is running. If you can not get the furnace to reset then this limit may have been tripped too many times and it is now indeed bad, but you still need to address the issue with it being tripped repeatedly. You can find a replacement limit at any supply house. Just make sure that temperature range I discussed is within 10 degrees either way and that the rod that sticks into the heat exchanger is the same length.