Unit installed Sept. 10, 1992. Lennox Pulse21 model# G21Q4-60-1, S/N: 5892C19536, Central Air Lennox Model# HS22-261V-2P, Serial# 5192E07200, 2-ton. Humidifier April Air Model 550 S/N: 104F12-191939. Honeywel Duct Mounted Electronic Air cleaner Model# F50E. I'm still having problems with the furnace not staying on all the time. Most of the time it will run, but once in a while you will hear it start up and 20-30 seconds later it stops running. I'ts on the fourth Jonhson ignition Control module. this one was put in about a week ago. I found the connection on the sensor to be loose and corrected that, but I'm still having the same problem. The only thing I haven't tried is replacing the flapper valve. How do I know if the valve is bad? I also haven't had the furnace pressure tested. Is it required and why?