My 8 year old Trane Model tuh2d120a9v5va furnace is not giving out heat. The service people are all busy due to the recent storms. Through the little glass viewing window I see red and green flashing lights. The red flashes 4 times, the green 3 times, and these overlap. The panel inside says the 4 flashing red means an open limit switch.

Where do I go from here?

Update: A 1" flat capacitor in the fan control module was fried black and cracked. A new module and $1000 later and the heat works again. Perhaps the power fluctuations due to the storm here caused some spikes that fried the module. I might put a surge protector on the line to the HVAC.

  • Check your filter, clogged filter can cause the high limit to trip. Mar 8 '18 at 18:07
  • 1
    Sell your old control module on eBay : others can fix it,.
    – Bryce
    Mar 9 '18 at 3:23

The high temp limit switches usually have a manual reset button that needs to be pressed to reset. I would try a reset it may have been a fluke that it tripped, if the reset dosent trip again it is probably fine. If it trips again you will want to verify the exhaust is clear, and the fan is freely spinning (some do need oil at each end to lube the bearings) low air flow on a high efficiency system can overheat the fire box.


There are 2 airflows in a furnace and you need to make sure both of them are clear. The first is the in house airflow, the cold air return bringing air into the furnace and the blower blowing it out through the vents. This is typically NOT the air flow that the lights are indicating are blocked. The second airflow is the air coming into the fire box/combustion chamber, providing oxygen to the burner and then the exhaust blowing out. This IS the air flow the error lights are talking about. When you get 4 blinking lights and the combustion chamber switches off automatically is may be as simple as wasps building a nest in the intake pipe during the summer. In the fall when the furnace turns on, the nest gets sucked in a blocks the air from getting to the combustion chamber. Typically, this pipe can be gently but firmly lifted straight up from the furnace. There is typically caulking around it to secure it and you will be breaking the caulk when you lift it up, this does NOT harm your furnace. But make sure it is not harming your roof. Most of the time it is connected to a flexible exhaust run in your attic but rarely it is indeed going straight through your roof and you may need to caulk your roof after this procedure, so check the attic before you do this.
Once you life the pipe, get all the wasp nests out (there will be several) and then put it back together. 9 times out of ten this fixes the problem. GLTA!

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