The Lennox furnace in my home is 11 years old. Some water leaked into it causing a faulty disc style limit switch. My furnace would run for only short periods of time and would not heat my home. Multiple things could have been the culprit. I began to triage the issue by checking the thermostat and air filter. The thermostat had been set at 68°F, yet the house would not warm up even with the furnace running all day. I replace my air filter every month. I checked the air flow direction arrow on the air filter and made sure it wasn’t plugged up. Finally, I cleaned the flame sensor. None of these steps corrected the issue.
After inspecting the internals of the furnace I noticed some oxidation/corrosion on one of the sensors. I could tell water had leaked down into the furnace… I assume from some plumbing work I had done earlier this year during the warmer months when I wasn’t heating the house. I also took note of the diagnostic lights after observing the furnace after power cycling it a couple times. I kept getting the same diagnostic code, which referenced the limit switches as the culprit.
After some online searches I learned the diagnostic code I was getting code indicate any number of problems, which wasn’t super helpful. I had already checked and cleared the easy things. With this knowledge in mind I decide to order several replacement parts. There were several online resellers to choose from with prices varying only a few dollars per part. I ended up ordering all the replacement parts from SupplyHouse.com. First, they had all the parts. Second, their pricing was better or on par with other suppliers. Third, their website and ordering process was more professional looking/polished. Fourth, I found their personnel to be very helpful identifying equivalent replacement parts. On this point, Lennox had superseded a couple parts with new equivalents that no longer shared the original part numbers. The cost of the parts with shipping and tax was approximately $175.00.
It took 2 days for the parts to arrive. I proceeded to repair my furnace by replacing the control board, flame sensor, and two different styles of limit switch. After a week of use I can say that the repairs were a success! My furnace has been heating our home as it should be.
FWIW, I am not an HVAC professional, electrician, or plumber. I simply enjoy the process of taking machines and appliances apart and figuring out how they work. Also, no one sponsored this video. I thought my investigation and process to repair my furnace may be helpful to others. My furnace is model number G40UH-36B-090-15.