reply to dm19891
Re: Furnace starts, then shuts off. My guess would be that the pressure switch is opening.
It may not necessarily be faulty, there could be an underlying cause. When the blower kicks on, it's pulling vacuum or putting pressure where there shouldn't be, causing the pressure switch to open.
You could very easily determine if this is the cause if you have a multimeter. Set it on to measure AC volts, and connect it across the pressure switch. Turn the furnace on, once the inducer blower is running you should read 0 volts (switch is closed). You should continue to get a closed 0 volt reading throughout furnace operation. If you read something different (such as 24V open) when the problem occurs, then you know that's what it is. Next step would be firguring out why the switch is opening, but that would be best left for a professional to figure out.
I'm gonna say flame sensor. Look at the burner furthest away from the ignitor. My furnace has 3 burners, ignitor on the far right, and the far left has the flame sensor. It's a probe that sticks down into the flame path to sense that the burners all ignited properly.
You can remove it and clean it with some emery cloth. That should bring it back into operating condition to test. If that works you may want to consider replacing it.