2001 Other Chevrolet Models air stops working after engine g

First thought is that the temperture door may be stuck. If the door is stuck between hot and cold then you get AC when you first start the engine, but once the engine gets hot hot air is mixing with the cold. To check this switch the door from hot to cold when you first start it. You should feel a difference between cooled air and luke warm air if you do the door is fine. If not, you will need to trouble shoot what is wrong. It could be stuck, or there may be an electrical problem. Another possiblity is that you may be low on refrigerant, or there could be a blockage in the system. Parts stores sell bottled refrigerant you can re-charge the system with youself, but a blockage would need to be fixed at a shop.