I figured out tonight that my my Lennox G24M heater works if I pinch the tube going to the pressure switch after the flame ignites. (The tube goes from the combustion air blower to the pressure switch.) The C.A.B. is the first thing running I think(?) when the unit senses a need for heat. Here is what happens when I don't pinch the tube...

Without pinching this tube, the unit fires up the gas and it runs for 5 to 30 seconds or so, then shuts off, in cycles. It does this for about 2.5 minutes then shuts off for about 5 minutes with LED 1 - off, LED 2 - slow flash, The panel says "Pressure Switch Watchgard: pressure switch open during operation or condensation drain blocked."

Everything seems ok if I pinch this tube, except the CAB and flue get very hot. (Blocked flue? Bad CAB?) I don't know if that's normal. I believe the heat exchanger is still under warranty for another 7 years. It looks like the pressure switch is easy to replace if that's the case.

Here is an online link to the G24M pdf file: http://www.ultimatehvac.com/LinkedDocs/Lennox/9723b.pdf

The trouble code is listed on page 32, 33, Problem #3. I greatly appreciate the help.