Troubleshoot the Lennox pulse. Part 2

This the second part of the Lennox Pulse G21 gas furnace troubleshoot. I cover no heat gas related problems.
This video is part of the heating and cooling series of training videos made to accompany my websites: and to pass on what I have learned in many years of service and repair. If you have suggestions or comments they are welcome.
If you are a homeowner looking to repair your own appliance, understand that the voltages can be lethal, the fuels are highly flammable and high pressures are used. Know your limits.

21 Replies to “Troubleshoot the Lennox pulse. Part 2”

  1. Worst mistake I ever made was replacing my pulse furnace.
    It functioned perfectly for 28 years. If I had it to do over I would replace the blower assembly (ran 24/7 and never failed)
    The circuit board and all ancillary controls and run it another 28 years. The exhaust was litterally COOL from this unit. The 90 plus Lennox I replaced it with exhausts room temp air with a TON of moisture. The pulse didn't exhaust a bunch of moisture. It was obviously much more efficient than the new furnace and my utility bill reflects that.

  2. Thank You for the helpful videos. I have a G14. My light is off on the board but it still runs normally most of the time. When it acts up it will fire and run for about 30 sec. Then shuts the burner off but leaves the fan running for about 30 sec. Then fires again. Then it repeats this until I manually throttle back the gas valve until the "pulse" so to speak flutters almost stopping, then I fully open it. After that it runs fine for a while. I just changed the air flapper but the gas flapper hasn't been changed in the 12 years I've been using it. Could the gas flapper cause this type of behavior?

  3. Where is the relay for the blower? My fan motor stays continuously until I wrestle off the blower/filter cover and while doing so I can hear the relay click open, then the motor stops until the next on/off cycle commanded by the thermostat.

  4. So I can hear the spark plug working. And there’s power to the gas valve, is it safe to presume it’s the valve? The LP tank is at 55% and tank valve is on.

  5. Lennox Pluse (10/1987) G1404/5-100-6 still chuging along. Replace flapper about every 4 years. Had to change Fan Limit switch – HOWEVER generic limit switch cause Blower to short cycle. So had to add Delay -Off-Relay set to 4 minutes (that feature was built into original fan limit switch).

  6. Very informative. With a G14, heats and fan runs, but burner sometimes goes out for 15 seconds or so, then lights again without interuption of the heating process. What could one check for? Thanks

  7. Very helpful video, thank you for sharing! I was able to narrow it down to the control module, bought a replacement and my furnace is up and running again! Just thankful we aren't in the middle of winter.

  8. Hello,

    I have a Pulse that except for replacing the control module 7 years ago the thing has been a beast. But it's now got a glitch, which is the purge fan continuously cycles on and off when there's no call for heat.

    Any ideas where to start to look for the problem?

  9. thank you for these videos it helped a lot I just got my pulse 21 working again. thanks a lot

  10. What would happen if the intake and exhaust pipes were cut right at the roof line, so the exhaust air could be sucked into the intake pipe? Yes, this has happened. The solar panel installer chopped the pipes and put on a pair of elbows. And the solar panels are floating a few inches above, so the exhaust gas is probably collecting under the panels.
    I’m thinking monoxide gas is being sucked into the intake and snuffing out ignition to some extent. Maybe the air flapper is getting more particulits caught in it?
    The furnace is having problems but the damage was done nearly 2 years ago. The furnace got through one california winter without any signs of trouble.
    Yes, I am getting this fixed immediately. I’m interested to know how serious or dangerous the situation is.


  11. Thank you for these awesome and very helpful videos! My Pulse21 randomly shuts off. I can restart it and it runs fine for a few hours, sometimes a few days but will eventually shut off showing a flashing green light on the ignition control. I turn off gas, turn off power and then restart and it always lights fine. What do you think I should replace?

  12. I have Pulse 21 furnace, everything works but when it's off I can smell gas odor from the exhaust pipe outside, turning the igniter to off makes the smell go away. Any ideas?

  13. Great video, my question my pulse 21 turns on heats and turns offf , but comes back on a heats house in 3 mins , I just replaced Blue ignition box with an aftermarket blackbox, and I hear a bearing like noise running after Furance is off, Thks

  14. What causes these dinosaurs to operate with the hum coming from the air flapper? Lennox parts + only sells the air flappers in 10 packs for 70 bucs and I dont want to charge somebody for 10 of them and it not be the problem. I work on commercial refrigeration equipment, split and self contained equipment, c02 refer. I've never seen or heard of these explosion furnaces.

  15. I can't thank you enough Grayfurnaceman! With your help I've fixed my furnace 4 times now. You should put up a donate button so people like me can support your work. You've easily saved me a thousand bucks.

  16. Great tip on checking the flame sensor current. Do you have any specs on what this should be (e.g. new/clean flame sensor, min acceptable?). Is the ~5uAmps you measured in the video enough current from the flame sensor to satisfy "flame on" for the controller?

    Update: I found that the threaded-on coupler for the wire-connector on my flame sensor wasn't even finger-tight. I put a small wrench on the hex portion (took a lot of fiddling just to this) until it stopped turning but didn't torque it. After this I was getting about 1.6uA at start-up and had it fluctuating up around 2uA +/- 0.2uA after a minute or so of running. Apparently that's enough to keep it going, but suspect it has more carbon build up than yours. I was able to get an 11/16" deep well socket on it, but couldn't get my hand on the wrench well enough to torque it loose.

  17. Hi! I have Lenox Pulse furnace. it turns on and heats for about 10 minutes then turns off for 1 minute and then keeps coming back on for 1 minute again and turning off after 1 minute repeatedly. I even changed the flame sensor with 2 different replacement sensors from 2 different companies but it still does not work. I cleaned the air flapper but nothing changed. I even later hired someone and he said he has no idea. I would appreciate some advice. I watched your videos but did not come across such a situation. Thank You!

  18. Check out the playlist: Furnace fan switches. If its a G16 it probably uses an electronic control located behind the thermostat terminal board. If its a G14 it probably uses a heat operated fan switch located on the upper left. Hope this helps.

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