The integrated furnace control is equipped with diagnostic LED. The LED is lit continuously when there is power to the control, with or without a call for heat. If the LED is not lit, there is either no power to the control or there is an internal component failure within the control, and the control should be replaced. If the control detects the following failures, the LED will flash on for approximately 1/4 second, then off for 3/4 second for designated failure detections.
1 Flash: Failed to detect flame within the three tries for ignition.
2 Flash: Pressure switch or induced draft blower problem detected.
3 Flash: High limit or auxiliary limit open.
4 Flash: Flame sensed and gas valve not energized or flame sensed with no “W” signal.
5 Flash: Overtemperature switch open.
Can you verify the (vent) spill switch is closed or open? It's usually found on the venting where it leaves the furnace. It would be a manual reset type. There may be venting problems. What kind of furnace is it? Is it a mid-efficiency vented up a metal chimney? It could be a blockage in the venting. Maybe the chimney liner has collapsed. I would look into this problem closely as reseting the switch and walking away could have serious consequences.