Air conditioners are sealed systems in which freon is used to create cool air. The freon is meant to last forever, as long as the system holds up and no leaks develop. But the reality is that cracks can develop in coils and other parts of an AC's system. When they do, freon starts to leak. This causes the AC to stop cooling properly. There are clues that indicate your AC is leaking freon and needs to be repaired.
Feel the vents to see if cool air is coming out. If you have a digital thermometer, hold it up to the vent and check the temperature as the cool air comes out. It ought to be blowing cold air in the 60 degree Fahrenheit range. If it isn't, it can mean that the freon is low. There are a few things you can check to be sure.
Turn the system off and let it sit for an hour or more. This will allow it to thaw if the system is frozen. The system can freeze up due to low freon or lack of proper air flow.
Check the vents to make sure they are open and replace or clean the air filter. This ensures that air flow isn't the issue if the system is frozen.
Turn the unit back on and check the air flow again. If it is cool, let it run for half an hour and see if it slows back down. Continuing to freeze when there are no air flow issues, indicates that it is a low freon issue.