Troubleshooting an air conditioner starts with making sure the thermostat is powered on and connected by setting the system to cooling mode and turning the fan on. If this fails, the power cable and circuit breaker should be inspected.
Safety switches can cause an air conditioner to stop running. In some systems, the condensate tray can overflow, causing the system to shut down. While this may indicate a bigger problem within the system, removing excess water can at least temporarily get the system up and running again. Doors are common on air handlers; ensuring that the door is closed may fix the problem.
If the air filter is especially dirty, replacing it may help. Fans that fail can also cause the system to not work, and almost all thermostats can set the system into fan-only mode. If the unit does not respond, the fan motor may be broken or the fan may be obstructed.
If the problem is in a basic component of the system, diagnosis may require expert help. If the fans run but cold air is not produced, the problem likely lies with the refrigerant or compressor. It can be difficult to know if the refrigerant level is too low, but signs of leaks are often easy to find. If the system produces cool air but cycles run for longer than normal, the compressor may be operating inefficiently.