Is there air coming out of the floor or ceiling registers?
If there is not, then, check to see if the fan on the outside unit is running. If the fan on the outside unit is not running either, check to make sure that the breakers for the furnace/air handler and the air conditioner are on.
If the breaker is not tripped, then the problem may be your thermostat, the furnace/air handler control board, or the wires between the furnace/air handler and the thermostat.
If your unit has a condensate pump, check to make sure the reservoir is not full. If the reservoir is full, a limit switch will normally stop the air conditioner from running. You will need to repair or replace the pump.
If the breaker is tripped, you can reset it by shutting it off and then turning it back on. If it trips again, then check the wiring in the furnace/air handler for signs of damage and repair as necessary.
If the blower is running but the fan on the outside unit is not, then continue your air conditioner troubleshooting at the outside unit.
Is the fan on the outside unit blowing out hot air?
If the blower isn't running but the fan outside is, the problem could be the blower motor, it's run capacitor, or the furnace/air handler control board.
If the fan motor is not running, then shut the power off to the unit. Then check all wire connections for signs of damage such as burning and repair as necessary. You should also check the wire connections in the compressor terminal box and repair if necessary.
You can use our capacitor testing guide to check the unit's capacitors and replace as necessary.
Continue your air conditioner troubleshooting by turning the power back on to the unit and observe whether the contactor closes or not. If the unit does not start, look for a reset button. Some units have a high pressure cutout switch that can be reset. Then, use a voltmeter to check to see if you have power to the unit and 24 vac to the contactor's coil. If you do not have power to the unit, you should check the wiring between the electric panel and the unit. If you do not have 24 volts to the contactor's coil, check the thermostat and the low voltage wires from the furnace/air handler to the condensing unit. If you have power to the unit and the coil on the contactor but the unit does not start, the unit's contactor may be bad.
If the compressor starts but the fan does not, the condenser fan motor could be the problem.
If the condenser fan starts but the compressor does not, the unit may be low on refrigerant, the compressor might be too hot, or the compressor might be bad.
Do you see ice on the lines connecting the indoor and outdoor units or is there water around the furnace?
If the blower on the furnace is running, check to see if you see any frost or ice around the units. You might also notice water around the furnace. Is there ice on the copper lines going outside? If yes, then shut the outside unit off for a couple of hours but let the blower on the furnace run. This will allow the ice to melt.
After two hours, you can continue your air conditioner troubleshooting. Turn the ac back on and let it run for about five minutes. Now feel the larger copper line at the outdoor unit. This should be cold and have condensation on it. If it is not cold or if frost is forming on the line, it is probably a freon problem. You will need to call a repairman because of federal refrigerant usage regulations.
If there is no ice or frost, then continue troubleshooting at the outside unit.
Air Conditioner Troubleshooting, Insufficient Cooling?
If the condenser fan and the compressor both run but the cooling effect is not adequate, you should check the temperature drop of the system. **NOTE** The unit should be operating for 10-15 minutes prior to taking temperature measurements. To measure the temperature drop, measure the air temperature leaving the air handler and subtract it from the temperature of the air entering the air handler. The result should be about 15 degrees for a high efficiency unit and about 18 to 20 degrees for an older unit (SEER less than 10). If the temperature drop is significantly lower than that, the problem may be the refrigerant charge.
If the temperature drop is significantly higher than that, then your air conditioner troubleshooting should focus on the system's air flow. The evaporator coil, filter, or blower wheel could be dirty. This is usually the result of not performing routine maintenance on the system. It could also be caused by a duct system that is too small. This is usually the result of improper design or installation. You may be able to increase the blower's cooling speed or replace your existing blower motor with a variable speed motor to overcome that problem.