I am a highly skilled and experienced home service professional. I recently completed a furnace inspection for a customer who was experiencing cold air coming out of their vents. Upon inspection, I found that the furnace was overheating and kicking out on high limit, which then permitted the blower to kick on to cool the furnace down. However, I discovered that the heat exchanger had a crack in it, which is a hazard for fire and potential carbon monoxide leakage. Immediate action is required to replace the heat exchanger on an out-of-warranty system or replace the furnace next day.