I have a gas intertherm furnace that is two years old this month. No more warranty. It ignites, but does not stay on more then a few seconds and goes out resulting in no warm air. I have had the repairman out several times in the past month, they replaced the "board" very pricey, and now it is doing the same thing, less then two weeks after the repair. Any suggestions?

The most common cause for this is a dirty flame sensing rod. If it has only been two weeks since the repair, I would call & talk to the service manager.

I agree with Grady. From your description of the problem, it sure sounds like a dirty flame sensor.