Last night our Trane XB80 lost its heat, but the fan was still working. After doing some research, I figured it was either the hot-surface ignitor or we had a clog in the condensate drain. I hooked a wet/dry vac up to the drain outlet and turned it on, but got nothing. I removed the panel around where the outlet pipe attaches to the furnace so I could find the other end of the drain and with my finger over that hole, turned the vacuum on. I had good suction at that end, so I have ruled out a clog. This leaves me with a bad ignitor, which doesn't surprise me. From what I have read, these have a life expectancy of 3-5 years which would put ours close to the outside of that range. During my research on the Internet, I also found some info about the LED light that could flash error codes. When I pushed the switch back in with the panel off, all the light did was blink about once a second with no variation. Am I right to believe that we could have a bad ignitor? According to the Trane manual that came with our furnace if the ignitor is bad the blower won't come on. Ours does. The only option in the manual for a problem of no heat, working fan was the clogged condensate drain. Any ideas? Its about to be a chilly night, and I want to keep them to a minimum! Thank you in advance for any help.

I finally managed to get my wife to help me for a few minutes, and I was able to get the LED codes. With the panel off and the safety switch held down, my wife turned the furnace on. When it got the call for heat, the LED flashed four times, paused, flashed four times, paused, etc. Per the label on the panel, this is an Open Limit Device. I jumpered across the limit switch, and damned if the furnace didn't fire right up. So, looks like we get to replace the limit switch today. Could have been worse.

We're pretty sure the reason it failed was due to lack of airflow. We had forgotten to change our filter, and it was pretty bad. That's the only thing we can come up with. Everything else checks okay.