Checking the air conditioner's power supply, fan, thermostat and filters are the usual steps for diagnosing a problem with the unit. Subsequently, you should check for blockage around the condenser, evaporator and drain lines. If no problems are detected in these areas, professional assistance may be required.
Air conditioners typically have two electrical power sources. The first troubleshooting step is to check that the air-conditioner unit and fan are receiving electrical power. Locate the main house breaker box or fuse panel, and check for a thrown breaker or blown fuse. Reset any tripped breakers and replace blown fuses. Next, check to make sure that the thermostat has power. Some thermostats are directly wired to the house electrical supply, while others have batteries. Replace batteries as needed.
Set the thermostat to air conditioning, and at 3 to 5 degrees below the room temperature indicated. Listen for the fan to kick on and locate a vent. If the airflow from the vent is weak, check the filters. Typically, they can be located behind the return-air vent cover.
Inspect the outside condensing unit if the airflow is strong but not cold. Clean grass, leaves and debris from around the outside of the unit. Clean the evaporator and the drain lines of any blockage. If the system is still not working, you may need to contact a professional service.