My Lennox Furnace (not sure of the model, but it has a SureLight board) went out a few weeks ago. It is only a few years old. I just fixed it and wanted to post the answer here, so someone else could find it if they have the same trouble.

The inducer fan would kick on, the igniter would glow, the gas would turn on and light-up for about 3 seconds and then it would all shut off and immediately do the same thing again. Sometimes it would stop and blink the error code for the pressure sensor was tripped. Blinking code sounds like a bad pressure sensor, but the time when it shuts off looks like a bad flame sensor.

The answer: the fan draws a vacuum on the pressure sensor when it is running. A small tube connects the fan to the pressure sensor. A small amount of corrosion was found on the fan end of this metal tube. The corrosion was enough to pull some vacuum, but not enough vacuum when the positive pressure of the burning gas passed the inducer fan. I scraped the corrosion out with a very small allen wrench and now the furnace runs like a champ.

I hope getting this on the net helps someone else like me. Thanks.

Thanks for the explanation. I'm sure it will be a help to many.

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Yes, you really have to find out the MAKE and MODEL to get good answers. There IS more than one machine made.

The Lennox model number for my furnace is G40UH-48C-110. Thanks.