AC not turning on? Check these before you call

Sort the problem into one of two cases first, because they have different causes.

• Nothing runs. No indoor fan, no outdoor unit, no click. Power or the thermostat signal is missing.
• The indoor fan runs but the outdoor unit does not. Power and the thermostat are fine, and the fault is at the outdoor unit or in the wire that tells it to start.

A blank thermostat belongs to the first case. The thermostat may have dead batteries, or the system's low-voltage power may be out, which also blanks many thermostats. The guide to a thermostat with a blank screen covers that branch in detail.

1. Thermostat setting, schedule or batteries. The simplest cause and the most often overlooked.
2. Tripped breaker or switched-off power. The furnace switch looks like a light switch and gets turned off by accident.
3. Condensate float switch. A clogged drain fills the pan, and the safety switch stops the AC before water overflows. Clearing the drain restores operation.
4. Blown low-voltage fuse. A small fuse on the indoor control board protects the thermostat circuit. It blows when a wire shorts, often the thermostat cable at the outdoor unit.
5. Failed capacitor. The outdoor unit hums or clicks and never starts. See what a capacitor replacement involves.
6. Worn contactor. The switch that feeds power to the compressor and fan has pitted contacts or a failed coil.
7. Failed transformer, control board or motor. Less common, and found by meter testing.

A technician traces power in order. First line voltage at the indoor unit and the outdoor disconnect, then the low-voltage supply from the transformer, then the cooling signal from the thermostat to the outdoor contactor. That sequence finds the break quickly.

If the fuse on the control board has blown, a good technician finds the short that blew it before replacing the fuse. At the outdoor unit they test the capacitor with a meter and inspect the contactor. These are fast repairs when the parts are on the truck.

If the float switch was the cause, ask the technician to clear and flush the drain line fully and to check why it clogged. A drain that clogs once tends to clog again without cleaning.

Everything past the thermostat, breakers and switches involves live voltage or stored charge. Leave the panels closed. You can still help the diagnosis by telling the technician exactly what happens when the thermostat calls for cooling: silence, a click, a hum, or the indoor fan alone.

A system that will not start is rarely a replacement case. Fuses, capacitors, contactors, float switches and transformers are small parts. The exception is a compressor that is shorted or seized, which shows up as a breaker that trips the moment the unit tries to start. If that is the diagnosis on an older system, compare the repair with a new unit before deciding. The AC repair overview explains how to frame that choice.