hi and good after noon, how old is your Lennox unit?
I haven't seen a reply form you on this chat so I will set us up as Q & A , it works better;-)
30 years old - cleaned five years ago
If the thermostat is calling for heat and the burners are ignited and the motor/fan sounds as if it is running there is no warm air being blown through the vents!
thermocouple is working OK gas ignites the fan/ motor seems to be blowing but no warm air comes from vents.