I have a newer (2007?) Trane XL90 (model: tdx2b080a9422aa)forced air gas furnace. The problem I am having is the burner and fan will not start. The thermostat is set at 67 degrees and shows it is calling for heat but no heat. Noticed it started coming on and off intermittently over the last few days. I took a multimeter to the ignitor and its not reading ohms. Have not checked the flame sensor yet. Should I? Also the fan is on on the thermostat, however it will not start on the furnace. Is this a startup sequence issue? the fan should start first I believe. Any help would be greatly appreciated.
I have the same issue... I did the filter change, checked all the thermoswitches then it worked for a few hours then no heat LED flashing 4 times. Then I measured the voltage from the 24V transformer and the voltage would vary from 11V to 19V max. Replace transformer and furnace is working perfectly.
Yours may be different, but generally high temp limit switches look like this:
Order the correct replacement online (if you can figure out the right one to get and have verified it has failed by testing for broken continuity) or take it to your local appliance parts supplier. Bring the model number of your unit with.
"well 4 blinks is an open limit or roll out" -Matthew
What else is on the same wire? (test the other parts of the safety chain)
After replacing a faulty ignitor, the elusive culprit for the OP was old batteries in the thermostat.
"I replaced the batteries in the thermostat, even though it wasn't calling for new batteries and now it works".
My Trane XR90 gas furnace was not producing heat. The red diagnostic light was blinking 4 times, which means "Open High Temperature Limit Device".
I did some analysis and found that I have two flexible condensate hoses that drain into two 3/4 PVC pipes. These PVC pipes meet at a Y junction and drain into one PVC pipe that leads to the exterior. The plumbers clay around one of the flexible hoses was old and crumbling, allowing air to enter. This was creating a backdraft, which prevented condensate from draining and causing a sensor to trip the unit. As soon as I removed the old, crumbling clay and replaced it with new, pliable clay, the unit starting heating again. Hope this helps someone.