Hi there everyone I work for a lennox dealer and I thought I would fill you in on some of the issues we've had with the Lennox G51.
Well let me start with propane aplications. Depending on the quality of propane, the pressure switch port on the bottom left hand side of the collector box seems to get pluged up with a white residue (maybe sulfur). It seems that you need to take of the colector box once a year and wash it out.

On the G51-1 series, any ventor motor that has a black wheel will more than likely crack in the first 5 years (material issue)

On the G51 with the updated white rodgers surelight board (either a dash 1 or dash 2 on the part number of the board not the model nuber of the furnace), we've had issues with the blower relays not closing and the pressure switch relays not closing.

Lennox has adressed the issue with the ventor motors by issuing a voluntary recall (good for one year), however they havent adressed the issue with their newer (cheaper) surelight boards.

I havent had any recuring issues with their G61mp or G61mpv models other than the pressure switch port being pluged in some cases when being used with propane.

Their new G71 units in my opinion are too advanced and the end consumer will never see the true benifit of thier aplication when compared to the operation of the G61.