I am having a diagnostic "nightmare"... only due to lack of knowledge of the Lennox Pulse. This is a roughly '90 model. 80,000 BTU... The problem: Furnace lights for about 10 sec- 2 minutes... shuts down... It resembles a primary air problem, sounds of the fire sucking air. A previous tech just changed the ignition lockout module said it would fix it... The cust. Has it in her head it is in the T-Stat. I know it takes special people when working on this type of Lennox. I do not have the model # right now... but seems it was G14Q480. Best can remember. Any Ideas wouild be great. I have tried removing the intake flue just in case intake pipe was clogged... same problem. I am returning tomorrow to start fresh. I did not have a lot of time to get into it today. It also seems rather tough to get to the flame sensor? Seemed like you have to have little hands. Well... any help or direction would be great!!
Brian

Sounds like you may really need to try to get to that flame senser,, give it a good cleaning with steal wool. Is the filter clean,that could cause some over heating with bad circulation,does it give a code ? Does it keep on trying to fire up over and over again,or does it stop trying when not successful? Let us know

Yep... refires.. filter is good. It does not give a code I seen. It really doesn't run long enough to get limit switch hot. Refires over and over... does not lock out... so it does detect a flame... but does not stay lit long at all... I would guess average flame run time is 20 sec. It is possible it could be flame sensor... but hmm... There is a air filter, called air diagphram on the intake of the flue right at the base of the exchanger. It was very "sooty", removed and cleaned it thinking that was the problem. Hope this helps!

The pulse units should have the internal filter replaced by a trained person.(air flapper assembly).The air that pulls through this then mixes with the gas during combustion.It can start clogging up and not give the right gas/air mixure. The gas valve orfice has one as well that may need replacing if problem still exsist.

HI there I have same problem it doesn't fire up takes more then 3 time before start raining. How much cast to replacing Gas valve? I called lennox dealer they asking 325 dollars is that right price?

I think you should make sure the thermostat is not electronic. Some of them have parameters limiting burn time and number of burns per hour. Especially relevant if someone replace a mechanical/mercury thermostat with an electronic.