Hi guys,

First time post here. I have a Lennox G40UH-24A gas furnance. Today I.realized my room temp is low, the furnance stopped heating. I shut down the elec power, then turn it on. The furnance starts up properly, the LEDs shows normal operating. After it runs for several mins, it stopped. the LEDs shows "Primary/Secondary limit switch open". I let the furnance cool down for about 30 mins, turn the elec power off/on, the furnance did the same(started for 2-3 mins, then stopped). I checked the primary switch with my multimeter, it IS open. I short the primary switch with a jump wire, the furnance start to run normally (I'd guess the secondary switch is not open), it won't run if I pull out the jump wire. I wondering if I got a defective primary switch? or any other defective parts cause the trouble? ie air blow fan not running as powerful as it should.

I'm not sure what I should next. Please can Y'all help.

 

The burners create the heat, and the air handler (blower) moves the heat out of the furnace into the house. If the airflow is reduced, the furnace temperature will continue to rise to an unsafe level. That's the purpose of the high limit switch: to shut down the furnace if the temperature gets too high.

The first thing I would do is replace the furnace filter. If the filter is clogged, that will reduce the airflow, and can cause this problem. After changing the filter, if you still have the problem, check for any obstructions in the heat runs (e.g. several dampers closed off). If that checks out okay, then check if the blower motor is running the correct speed. I suspect that once you get the airflow back to normal, your problem will go away.