My Lennox 80MGF furnace was failing to ignite. After troubleshooting with the help of the web, I figured out that it was providing a "Limit Switch Open" error code. I used a multimeter and determined the primary limit switch was closed (no voltage across the switch), so I figured it must be an issue with the secondary limit switch. I ran a jumper between the 24V hot and return terminals, and the furnace immediately began its ignition sequence. The good part is that it continued firing after I removed the jumper.
I recycled the thermostat and power to see if it was fixed for good, and while the ignition sequence started, it soon stopped and I got the same old error.
I then disassembled the blower assembly to take a look at the secondary limit switches. I figured maybe there was something mechanical I could wiggle. Turns out that they are pretty much immovable parts that look like temperature sensors.
Anyway, I reassembled my furnace, and it's now working perfectly! I feel like a genius, since I've never worked on a furnace before. But I also know that I didn't really do anything except wiggle some parts.
Here's my question: Should I just stop now and be happy? Or should I preemptively replace the limit switches?
Don't worry Be Happy. It probably was a loose connection with the molex/wiring harness that the wires from those limits attach too. (I do lots of Lennox units). You jiggled/wiggled and they seated properly. NEVER have a failure of those switches. If it ain't broke don't fix it.
"Cut it twice and it is still too short".