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Old 02-05-2010, 12:18 AM   #1
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Replace secondary limit switches on Lennox 80MGF?


A bit of a story to set up my simple question...

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?

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Old 02-05-2010, 06:31 PM   #2
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Replace secondary limit switches on Lennox 80MGF?


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.

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