I have a Lennox G21 pulse furnace installed in 1993. The unit will start to pulse briefly - maybe 5 seconds or so, and then shut off. We've had two service calls recently. On one call they replaced a flapper, and on the other call did the pressure test, which passed. I've done a visual inspection, cycled power, etc. This sounds like a sensor to me. Any idea which one? Thanks.

First thing I would check is the condensate to see if it is plugged. On the bottom of the exhaust pipe there will be a cap or a plug that tends to get crap in it and plugs the inner small tube and will cause this. Secondly I would try a new ignition control. Being they are so expensive it would be a last try.