AC Not Turning On: A Homeowner Checklist

Check 1: The Thermostat

Confirm the display is on. If it is blank, replace the batteries in models that use them — a weak battery is a very common cause. Confirm the mode is COOL and the setpoint is below room temperature. On smart thermostats, check whether a schedule, away mode or software update has changed behavior.

Check 2: Electrical Panel

Look for a tripped breaker. Residential cooling systems typically have two: one for the indoor air handler or furnace, and a larger one for the outdoor condenser. A tripped breaker may not look obviously out of line — switch it fully off and then back on.

If it trips again immediately, stop. Repeated trips indicate an electrical fault in the equipment or wiring that requires diagnosis, and repeated resets can make it worse.

Check 3: The Outdoor Disconnect

Near the outdoor unit there is usually a small box with a pull-out handle or switch. It can be left out after service work or pulled by someone doing yard work. Confirm it is seated and in the on position. Do not open the box or touch internal components.

Check 4: The Indoor Unit Door Switch

Many furnaces and air handlers include a safety switch that cuts power when the access panel is removed or not fully seated. If a filter was changed recently and the panel was not reinstalled correctly, the system will be dead and the thermostat may be blank. Reseat the panel firmly.

Check 5: The Condensate Float Switch

If the condensate drain has clogged, a float switch shuts the system off intentionally to prevent water damage. Look for standing water in the pan under the indoor unit or in the secondary pan beneath an attic unit. This is one of the most common reasons a system simply will not start during humid weather.

Check 6: Look and Listen

If you smell burning or see scorch marks, turn the system off at the breaker and mention it when you call. Do not attempt to restart it.

• Does the indoor blower run even though the outdoor unit does not?
• Does the outdoor unit hum, click, or make no sound at all?
• Is there a burning smell or visible scorching?
• Has anything changed recently — a storm, a power outage, service work?

What a Technician Will Likely Check

• Voltage at the disconnect, contactor and control transformer
• Low-voltage fuse on the control board
• Capacitor condition and contactor operation
• Thermostat wiring continuity from stat to equipment
• Safety switch and float switch status
• Motor windings and compressor electrical readings
• Control board diagnostics and fault codes