Air conditioners come in two types: window units that cool a single room, and central units that cool a whole house. Either type can run for many years without problems, but if your air conditioner stops cooling or does not keep your place cool enough, you can check and potentially resolve several possible causes yourself before calling for repair service.
WIndow AC Troubleshooting
If your unit doesn't turn on at all, check that it's plugged into a live electric outlet, says the RepairClinic website. If the outlet is dead, check your fuses or circuit breakers. If the unit has power but it doesn't turn on, the problem should be checked by a repairman. If the unit fan runs but there's no cold air, check that the thermostat is set correctly. You also can check whether the compressor motor inside the AC cabinet is running. If it isn't running, or if it makes a continuous noise without cooling, the problem requires professional servicing..
If the unit is running but the air it produces doesn't seem cool enough, use a thermometer to check the temperature difference between the air going into the unit and the air coming out. If the difference is more than 15 degrees, the unit is running correctly. If the difference is less than 15 degrees, make sure that the air damper is closed, that any covering has been removed, and that the condensing coil on the outside is clean and free of debris. If the unit never shuts off, raise your thermostat setting a bit. If everything else checks out OK, your window AC simply may be too small for the room it's working to cool.
Central Air Troubleshooting
If your central air conditioning unit isn't cooling your house as it should, you should first check that your thermostat is on "Cool" and that it is set to a temperature lower than the curent room temperature, says the HVAC for Beginners website. If it's OK, check to see if the circulating blower and outside fan are operating. If they are, check for dirty filters, evaporator coil and blower. If blower and fan are not running, check the circuit breakers for the blower and fan. If they're off, reset them. If the breakers keep tripping, call for repair service. If only your blower or only your outside fan is running, you likely have a problem that needs professional service.
Look for Ice
If you see ice on the copper lines to the outside unit, you may have a frost block. Shut off the outside fan, but keep the furnace blower running for two hours to melt the ice. After two hours, turn the outside unit back on, wait five minutes, and check the larger copper line at the outside unit. It should be cold with condensation on it. If it is not cold, or if it has ice on it, you have low refrigerant and need to call for repair service. Also check to be sure your condensate pump is removing water from its reservoir. A failed pump will shut off your system.