Run your air conditioner for a few minutes now, before you need it. If you wait until the first hot day to discover is isn't working, you'll find yourself on a waiting list, sweltering sometimes for days before an air conditioning specialist can come to fix it. Change the filters regularly. Dirty filters restrict airflow, reducing efficiency and worse case, can cause the evaporator to ice up. Disposable fiberglass filters should be replaced. Electrostatic or electronic filters need to be washed regularly. Be sure all access panels are secure, with all the screws in place. Clean obvious obstructions such as newspaper, leaves, etc. from around the exterior of the unit. Be sure the thermostat is set in the cooling mode. Just setting the dial below room temperature will not activate the air conditioning if it is set in the heat mode.