Eight Reasons Why Your AC Won't Turn On-And What to Do 1) Check the Thermostat. ... 2) Check the Air Filter. ... 3) Check the Circuit Breaker. ... 4) Check the Indoor Shutoff Switch. ... 5) Check the Outdoor Shutoff Switch. ... 6) Identify the AC's Reset Button. ... 7) Inspect the AC for Ice Buildup. ... 8) Ensure the Condensate Drain Isn't Clogged. More items...