My LENNOX Gas Furnace-High Efficiency G51MP Series is 2 years old. The furnace used to work perfectly. 4 Days ago I started having a problem with it. We woke up in the middle of the night and the house is very cold. The thermostat is set to 72°F and it was reading 66°F. The furnace was running but no hot air.
I tried to reset the thermostat, open the furnace panels to see if something was stuck inside, checked the vent pipes going out of the house, the filter is new. Every thing was fine but no hot air. Finally I decided to shut the furnace off from the main switch ( It has a label "LEAVE ON"). Anyway I turned the furnace of and waited for an hour. During that time I drained the water from the condensate trap (on the side of the furnace).
After the one hour, I switched the furnace back on. The heat came on and the furnace was fine.
The following day the same nightmare happened. The furnace was working fine with hot air and all the sudden stopped blowing hot air while it is still running (I can hear the blower and see it when I open the cover)

Please give me your best advice and experience. :confused:

On the second time around did you drain the condensate trap again?

First thing to let you know is a G51 is still under warranty,this furnace has 10 year parts and labor warranty. So if to no avail contact your local Lennox dealer.

Assuming this furnace is in the upflow position, installed in the bottom door is clear plastic peek hole to view the furnace diagnostic code, next time it acts up count the flashes, write the code down, remove the blower door, on the inside of the blower door you will find a chart, compare the codes and post back, we will try and help.

First thing to let you know is a G51 is still under warranty,this furnace has 10 year parts and labor warranty. So if to no avail contact your local Lennox dealer.

Assuming this furnace is in the upflow position, installed in the bottom door is clear plastic peek hole to view the furnace diagnostic code, next time it acts up count the flashes, write the code down, remove the blower door, on the inside of the blower door you will find a chart, compare the codes and post back, we will try and help.

I have this same problem and my D1 and D2 lights blink slow alternately. I think the flame sensors are bad, any ideas?

Try cleaning the flame sensors.

Try cleaning the flame sensors.

They are the ceramic, round type.

Yes with metal tips of some time. Ceramic does not conduct or generate current by itself.

Did you ever get it to work. We are having the same problem with our Lennox G51MP. Any help would be appreciated. Thank you!

It sounds like the same problem that I am having with our 6 year old Lennon G51MP furnace. By the way my papers say that I have only a 5 year warranty. I have had the repair man out 4 times. Each time they poke arount everywhere and the flame comes on and they say it was a different problem each time. The last repairman said if it happens again, disconnect the red rubber tube connecting to the cold collector box and poke a wire into the hole to the collector box to clean it out. I did that this morning when the combustion blower continued to run without a flame, and sure enough the thing is heating again.
The repairman wanted to replace the cold collector box for $400 but would not even then guarantee that the problem waould be solved permantly.

Ours ended up working again after restarting a few times and emptying the condesate trap a couple of times. Thank goodness.