[mike n]

When the system shuts off it is in lock out the control board is reading the entire safety circuit and diagnosing the problem for you there is probably a flashing light that will tell you where to look (open pressure switch, flame failure, etc.)
Before you turn the power off look for a failure code look for a sight glass in the blower door it will give you a flash code then if the installer put the wiring diagram on the door like he was supposed to it will give you the legend for the flash codes (3 flashes flame failure, 4 flashes pressure switch open etc.) if you tell me the flash code I can tell you where to look