Why does my air conditioner keep switching on and off?

Modern inverter units slow down rather than stop as the room nears the set temperature, so a unit running quietly at low speed is doing its job. In heating mode on a cold morning, the outdoor unit pauses now and then to defrost its coil, and the indoor fan may stop for several minutes. That's normal. Short cycling is the compressor stopping and starting every few minutes, often with the room never quite comfortable.

An oversized unit cools a small room so quickly that it switches off before it has run long enough to pull moisture out of the air, so the room feels cold and clammy. The sensor in a wall split reads the air coming back to it, so a head mounted where its own air bounces back off a wall, bulkhead or cupboard thinks the room is done when it isn't. On ducted systems, a wall controller in a hallway, near a door or in afternoon sun misreads the house in the same way.

Dirty filters and coils, a blocked outdoor unit and low refrigerant all push temperatures and pressures out of range, so the unit stops itself to avoid damage, then tries again. On hot northerly afternoons, an outdoor unit squeezed into a narrow side path and breathing its own hot air is a common trigger on smaller estate blocks.

Starting is the hardest moment in a compressor's working life, so constant cycling shortens it and wastes power. It also leaves rooms swinging between too warm and too cool. Clean the filters and clear around the outdoor unit first. If that doesn't settle it, a technician can check the refrigerant, sensors and airflow, and tell you whether the unit's size is the real issue.