I have a 99 Audi a6 2.8. When I turn on ac it blows hot air and there is no snowflake on the instrument cluster. I bought one of those r134a can and when plugged into the low side it shows charged not overcharged or under. That is with the car off. When I turn on the car it show that its undercharged. I plugged the r134a can while it was on and tried charging it that way and the needle wasn't going up on the pressure gauge. I turn the car off and plug it back to see the pressure and its OVERCHARGED! If anyone can help me out or point me to the right article. Thanks in advance.

I released the pressure back to normal but it still blows out hot air

 

Auto Air Conditioner Blowing Warm Air, Not Cooling