Why won't my AC turn on?
Prices and facts checked July 22, 2026.
A dead AC usually comes down to power or a safety switch, not a broken compressor. Work in this order: dead thermostat batteries, a tripped breaker or blown fuse, and a full condensate drain that tripped the float safety switch are the three you can check yourself. If those are clear, the likely culprits are a failed capacitor, contactor, or transformer, which are pro repairs in the $150 to $450 range.
Check yourself, in this order
- Thermostat: if the screen is blank, replace the batteries. Then confirm it is set to COOL and the target is below room temperature. A blank or misprogrammed thermostat is the most common no-cost cause.
- Breaker and disconnect: check the indoor air-handler breaker and the outdoor disconnect box by the condenser. Reset a tripped breaker once. If it trips again, stop; a repeating trip is an electrical fault for a pro.
- Condensate float switch: if the drain pan is full or the drain line is clogged, a safety float switch shuts the system off on purpose to prevent water damage. A wet-vac on the drain line outlet, or clearing the clog, often restores power.
- Fuses on the control board or in the disconnect: a blown low-voltage fuse (often 3 or 5 amp) kills the system. A pro replaces it and finds why it blew.
What it is if the basics are fine
- The outdoor fan hums but will not spin: a failed run capacitor, $150 to $450 installed. Ask to see the reading.
- Nothing happens at the outdoor unit at all: a burned contactor ($150 to $350) or a failed transformer.
- Repeated breaker trips: a shorted compressor or motor. This is the expensive branch, and it deserves electrical readings and a second opinion before you accept a replacement quote.
Common questions
My thermostat is on but the AC won't start. What now?
Confirm the thermostat is set to COOL below room temperature, then check the outdoor disconnect and both breakers, then the condensate drain and its float switch. Those three cover most no-start cases. If the outdoor unit hums without spinning, it is likely a capacitor, which is a pro fix.
Why does my AC keep tripping the breaker?
A breaker that trips once can be a fluke, but a breaker that trips every time it runs is protecting you from a real fault: a shorted capacitor, a failing motor, or a shorted compressor. Do not keep resetting it. Call a pro and ask for the electrical readings behind the diagnosis.
What is the float switch on my AC?
It is a safety switch that shuts the system off when the condensate drain pan fills, so a clogged drain does not flood your home. A full pan or a clogged drain line is a common reason a healthy AC will not turn on. Clearing the drain usually restores it.
Keep going
Sources
- HomeAdvisor / Angi AC repair cost guides (2024) (capacitor, contactor ranges)
- ENERGY STAR maintenance guidance (condensate drain and airflow)