I have the same furnace with the same problem only when mine trips out everything stops except the combustion air fan, which continues to run indefenetly. If I kill the power and restart it will usually go. I checked the pressure switch which appears to be operating correctly and I had a Lennox service man out that double checked the pressure switch and installed a flame sensor. That solved the problem just long enough for him to charge me $110 and leave. What next? My ignition control module was replaced 1-1/2 years ago with an upgraded Johnson controls model. Any other ideas??