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.


  • Did this start happening after a change in the fan or duct system? Jan 3, 2018 at 4:30
  • No. If this is a reference to my other post about adding ducts off a trunk, this is at my buddy's house. The trunk change is my own house.
    – Steve Oaks
    Jan 3, 2018 at 5:25
  • Are you using an ohm meter on a live circuit? If so this could be why you are seeing a change.
    – Ed Beal
    Feb 7, 2018 at 14:13
  • @SteveOaks What was the cause of this? Was it the answer below or something different? Jul 11, 2018 at 13:33

3 Answers 3


The limit switch should always be at 0 ohms. Since it is bouncing around it sounds like a bad switch. Check the temperature at the outlet of the furnace and at the inlet of the furnace. Subtract the two to get you the delta T. Look on the name plate for the correct delta T for your particular furnace. It will always be a range such as 30-60 degrees. If you are over the max number you will need more air flow. The limit switch is much higher than this number however and you should not be anywhere near hot enough to trip the limit switch. If you are measuring over 140 degrees or so at the outlet it is likely the limit switch is doing its job and you need more air flow


I resolved the issue by reducing pressure on the duct system. I opened up some closed vents in the house. Buddy claims he had not closed any recently (to the issue starting). But we opened up multiple vents further and one that had been closed shut for 3+ years (without any issues).

My theory is the limit switch is getting old. Months after this issue, and the furnace running great, we went ahead and changed the limit switch for good insurance against more issues this upcoming winter.

  • 2
    Thanks for coming back with your answer to your problem! Nov 1, 2018 at 16:44

Possible defective holding coil in gas valve that once warmed up will open from heat expansion, and then the gas flow is interrupted when the coil opens and permits the valve to close, and shuts off the fire until it cools and closes after cooling down Then the furnace is recycled and the fire comes on again. Flame sensors rarely fail. It can do this erratically.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.