Found the pressure switch was closed when inducer motor was not running. Through further investigation found the pressure switch was full of water due to the drain on the air conditioning was plugged cause the drain pan of air conditioning to overflow on top of the burner box of the furnace. Water then filled burner box and drained through pressure switch tubing down into pressure switch. Flushed drain line with nitrogen, recommended customer also replace pressure switch due to having filled with water and not being able to guarantee properly consistent operation of pressure switch. Customer choose to wait on replacing the pressure switch at this time. Informed customer that since he was choosing to wait to replace the pressure switch at this time if we had to come back on a later date to replace it that he would have to pay another service call charge. Customer agreed and understood this. System is operational at this time.