Furnace (Lennox G40UH, 12 years old) runs for about 5-minutes (sometimes as high as 7-minutes) and they turns off despite the heating call from the thermostat continuing. Flame sensor has been replaced. All vents in the house are open. Removing the furnace filter and it’s cap (5”x16” cap)appears to buy about another 2-minutes of run time…though that could have been a fluke.
The High Limit switch is showing 0 ohms when the furnace is not in a heat call and for the most of the cycle during the heat call, sometimes there would be a few ohms (under 10) for a few seconds. On multiple shutoffs when the flame when out (blower still on) the High Limit switch jumped to an open circuit and then quickly to 142K ohms (photo below) and then stepped back down to 0 ohms over the course of a minute or two. When the flame was turned off in this way (aka not by the thermostat), the furnace would try to fire 1-3 times again before it was able to fire and stay on for the roughly 5-minutes again. When it tried to restart some resistance would appear across the High Limit switch.
Flame is a healthy blue with no signs of drafting from the exhaust venting.
When the furnace is unable to fire, it does signal a code for “Primary or secondary limit switch open. Limit must close within 3 minutes or unit goes into 1 hour Watchguard.”
Actions taken: - used compressed air to blow out dust from blower area, burner, exhaust fan, and exhaust venting pipe. - Replaced the flame sensor - Ensured vents throughout the house are open. - Tested removing allergenic 3M furnace filter. - Tested using a cheap blue furnace filter. - Tested using no filter.
Could the fact that I am running the furnace’s heating cycle back-to-back-to-back be causing the issue? For about 90 minutes I was running heat cycles typically with no more than 5-minutes in between.