If you attempt to run your A/C and it doesn’t work at all:
Make sure your thermostat:
Is set to “AC,” “cool,” or “fan”
Is set to at least 5 degrees below the current room temperature (if it isn’t, your cooling system sensors may not detect that it’s time to get working)
Check for tripped circuit breakers.
A circuit breaker trips (turns off) to stop the flow of electricity and keep a circuit from overheating; reset tripped circuits and try your air conditioner again (Tip: if it isn’t already, label your A/C and other circuits for quick reference during an outage).
Make sure all power switches are on.
Make sure power switches that affect both the outdoor (condenser) and indoor (air handler) units are on.
If your A/C is running, but not as efficiently or cool as you think it should, try these measures:
Clean air intakes and replace your A/C air filter – Dirt can cause a compressor or fan to overwork; keep air intakes clear and replace clogged HVAC air filters to ensure adequate airflow.
Make sure windows and doors are closed – Unnecessary overwork will certainly lead to inefficiency – and more than likely to costly cooling system repairs down the road.
Look signs of reduced efficiency and airflow – If you notice poor airflow from vents and higher than usual bills, it might be time to get your system checked. Improperly maintained equipment can lose 5-10 percent efficiency or more every cooling season. Duct leaks can also sap cooling performance.
Make sure your A/C unit is matched to your cooling load – If you’ve built an addition to your home and haven’t upgraded your air conditioner, it may have to work too hard to cool your space. The same unnecessary wear and tear can occur if your unit is oversized, since constant on/off cycling will tax the compressor and fan controls.
Check for refrigerant leaks – If your refrigerant is low, it’s probably due to poor installation or a leak. Either way, you’ll need a trained technician to service your cooling system. After testing the repair, your technician can recharge your system with the right amount of refrigerant.