I have a rheem classic 90 furnace.
The vent pressure switches act up, it seems there is a pattern to the problem, when it is very cold and windy the unit stops working, the problem indicator light flashes 4 times with a 2 second pause, according to the trouble shooting paper on the accesdoor the 4 flashes indicate the vent pressure switches.
To get it working i just have to remove the access doors, tap on the vent pressure switches and put the doors back on, then it works fine, stays working for anywhere from a few hours to a few months. Its a pain in the ass, sometimes it shuts off a few times a day. I checked around and was going to replace the vent pressure switches (about $100 and easy to do from what i read) but then i came accross some people with the same unit having the same issue during the same weather pattern. I found this forum thread, excuse me if this link wont copy and paste http://hvac-talk.com/vbb/showthread....-90-Plus-issue
According to that thread it sounds like its either a issue with this model or thepvc intake and exhaust vents...one suggestion was to remove the rubber cap off the side of the unit, that allows the unit to vent properly, some of the hvac workers advised against that for some reason, and then the mods on that site said they can not give out do it yourself suggestions, i figured id give tacomaworld a shot before waisting money, this site is like a "jack of all trades"