First I would go with Harold's suggestion if cleaning off the flame sensor. It's tricky to get to but there should be a wire coming from the ignition control module (blue or grey box on the right side inside the top access door). The wire is either red, yellow or has a white cloth like covering. It should be labeled sense on the ignition control module. The flame sensor is on the other end of this wire (oddly enough). It looks like a metal rod sticking into the flame. Take this out and clean it off. Use a small file like a spark plug file. Don't use sandpaper as this can leave a silica residue on the sensor. When this is cleaned and reinstalled it should work fine. If it doesn't check what type of ignition control module it is. If it is a robertshaw( dark grey in color) you should be alright. If it is Johnson control(blue) replace it. This model has always been trouble, Lennox knows it but won't issue a recall. Not really Lennox's fault. The Johnson control is much more sensitive than it needs to be. If it's not sensing a certain voltage it will lock out. The robertshaw is not as sensitive.