Looks like I'm part of same club here. We have two Pulse 21 furnaces that were installed in 1992. For some reason, the furnace for the main floor trips out as other describe. Usually this happens at low temps, maybe 15 degree F and below. Wind might agrevate too. The furnace for the upstairs has never tripped by the controller failed once. The furnace that routinely trips (fourth time this season just this morning, has been serviced many times. The technicians have replaced several items, including the controller, but no permanent solution. I Lennox specialist has visited twice, but has only made some suggestions.
I haven't been able to find out what causes the trips. My pet theory is that since both furnace exhausts and intakes are fairly close together, that maybe the downstairs furnace detects CO in the incoming supply air. Again, I don't know what trips are built into the controller.
Solution is always the same - run downstairs and flip the power off to the tripped Pulse 21.
We always have to have someone watch the house if we go away during the winter. We don't have this problem with the "upstairs" furnace, so hopefully the house won't get cold enough to freeze.