My LENNOX Gas Furnace-High Efficiency G51MP Series is 4 years old. The furnace used to work perfectly. We went away for the weekend set the thermostat at 62 degrees and came back to a cold house last night of 53 degrees and cold air coming out. We check the filter (brand new) and the intake is clear. The diagnostic code is saying that D1 (red light) is off and D2 (green light) is slowly blinking. We can't get it to work consistently. We turned the heat off to come to work so we would would not hurt anything further. We got it to go back on last night before we went to bed, then when the heater kicked on this morning it just blew out cold air. Please help.

After emptying the condesate trap and resetting the furnace a few times we got it to work fine. We called a Lennox technician and that is what the told us to do. We didn't find anything clogging the condesate trap so it is still a mystery, but at least it is working again!

It doesn't take mutch of a restriction to keep a pressure switch from closing. If your condensate drain goes to the outside make sure its not freezing and backing up. Im glad you got it up and working.