My air conditioner had been working fine until a few days ago and a friend checked to see if the freon was low and it was. He took his gauges and put some freon in it. It worked fine for a couple of days and then today when I came home, the EM HT red light was on and the vents had warm air blowing out of them. What could be the problem. The unit is about 17 yrs old. No problems till now. Thanks for your help.