I have a Trane XR80 furnace, with a self-check flash code system (flashing red light). The furnace has been blowing cold air when turned on, and not heating up. The red light flashes 4 times, indicating that we have an "open Temperature Limit circuit". I believe that this means that our temperature limit switch is not working correctly, or there is some other reason that the limit switch(es) are open, rather than closed. My thoughts are that the switch must be closed in order for the operation to proceed to whatever it takes to heat up the air. I would personally like to change the Temperature limit switch, which looks like a very easy task (according to the photo of the temp limit switch). Am I correct in my evaluation of the problem, and does anyone know How to change the switch?
Thank you

Just because a limit switch is open may not mean its bad. It may be open due to lack of air flow - dirty filter, dirt cooling coil (evaporator coil),dirty blower wheel,and so on and so on. Check before you replace it, you may save some money. A limit trips for a reason. Over 90% of the time its doing its job. Give us some more info and let us know how it turns out.

Thank you, T-Top for your answer. We finally just called the company that sold us the furnace. They came out and said that it was the circuit board. He showed me the brown burn spot on the board. When I mentioned that the self diagnostic red switch was blinking 4 times (which means open temp limit circuit), he said that when the circuit board has a probem, it can send any signal, and that it may be an incorrect signal, since the board is malfunctioning. Luckily for us, the system is only 3 years old, and it was, is, still under warranty, so no costs! :-) SO, I guess the lesson learned is to check the circuit board before anything else!

Mine just had the same 4 rapid flashing light on my American standard 1.5 year old furnace. After a quick call to the service tech, he said to tap the back side of the square brown sensor mounting (located just under the burners) with a screwdriver. That did the trick and the furnace is running fine. That sensor will engage if an over-heat condition is present. The excess heat was caused by a dirty air filter.