AC won't turn on: what to check before you call

What to do, in order

1. Check the thermostat: set to COOL, fan AUTO, target temperature at least 3°F below room reading. Replace dead batteries.
2. Check the breaker panel for a tripped AC breaker (usually a double-pole 30–50A). Flip fully OFF then ON.
3. Look for a float switch in the drain pan or on the air handler — a clogged condensate line lifts the float and kills the system. Wet-vac the drain line from outside.
4. Replace a clogged filter. Severe restriction can trip limit switches and freeze the coil.
5. Listen at the outdoor unit: humming but no fan = likely capacitor; total silence = power or contactor issue.
6. Note the outdoor unit's model plate before calling — it speeds up the quote.

Red flags — stop and call a pro

• Burning smell or scorch marks at the disconnect box — kill power at the breaker and call now
• Capacitor visibly bulging or leaking oil — do not touch terminals; capacitors hold a charge
• Repeated breaker trips after reset — stop resetting and get it diagnosed
• Water dripping through the ceiling below the air handler

Prevention

Replace filters on schedule, pour a cup of white vinegar down the condensate line each spring, and have the capacitor and contactor checked at annual tune-up — they're wear parts with a 5–8 year life.

This guide is maintained by the ee.ltd editorial team — reviewed for 2026. Browse all home repair guides or compare hvac costs.

Related guides

• AC running but not cooling: 6 checks before you call
• Furnace short cycling (starts and stops every few minutes)
• AC evaporator coil frozen: why it happens and the thaw protocol
• Thermostat not working: dead screen, wrong temps, no response

What this repair typically costs

• AC repair cost guide
• AC replacement cost guide
• Furnace replacement cost guide
• Heat pump installation cost guide