5 Reasons Your Aircon Keeps Turning on and Off

Normal aircon operation follows a cycle: the compressor runs until the room reaches the set temperature, then the compressor stops while the fan continues. After the room warms slightly, the compressor starts again. This cycle typically lasts fifteen to twenty minutes or more depending on the room and heat load.

Short-cycling compresses that cycle into a few minutes or less. The compressor starts, runs briefly, shuts off, and restarts. Each startup draws a surge of current — far more than steady-state running. Repeated surges wear the compressor windings, stress the capacitor, and spike electricity consumption. Left unchecked, short-cycling can shorten a compressor's life significantly.

An aircon with too much BTU capacity for the room cools the air around the thermostat sensor very quickly. The sensor reads the target temperature, tells the compressor to stop, but the rest of the room has not actually stabilised. Within minutes the sensor reads a rise and the compressor starts again. This rapid cycling is a design mismatch, not a fault — the unit is working exactly as intended, just in a space too small for it.

This is most common after a renovation where room sizes changed or when a homeowner upgrades to a higher-capacity model without recalculating the load. The fix is either to use the unit in a larger space or to replace it with a correctly sized one. No amount of servicing will resolve an oversizing problem.

When the evaporator coil is coated in dirt, its ability to absorb heat drops. The coil surface gets colder than it should because the heat exchange is impaired, and ice can form on the fins. Ice further blocks airflow, which accelerates the problem. The compressor sees abnormally low suction pressure and the overload protector shuts it down. After a pause, the ice melts slightly, pressure recovers, and the compressor restarts — only to repeat the cycle.

A clogged filter causes the same airflow restriction. The coil starves for warm air to absorb, gets too cold, and the protection circuit kicks in. This is the most common and cheapest-to-fix cause of short-cycling. Cleaning the filter and washing the coil during a general service or chemical wash often resolves it completely.

Refrigerant absorbs heat at the indoor coil and releases it at the outdoor coil. When the charge is low — usually because of a slow leak — the suction pressure drops below normal. The compressor's low-pressure protection trips and shuts it down. After a rest, the pressure equalises enough for the compressor to restart, but the underlying shortage has not changed, so the cycle repeats.

Low gas and a dirty coil can produce nearly identical short-cycling patterns. The distinction matters because topping up gas on a dirty coil masks the real problem, and cleaning a coil when the gas is low does not fix it either. A technician who measures both suction pressure and coil condition can tell which factor is dominant — or whether both need attention.

The thermistor is the temperature sensor that tells the PCB (control board) what the room or coil temperature is. If the thermistor drifts out of calibration, it sends incorrect readings. The PCB may interpret those readings as the room being at temperature and shut the compressor down prematurely. A few minutes later, the actual temperature triggers a restart.

A PCB fault can produce the same behaviour if the control logic malfunctions. The board may send a stop signal to the compressor at random intervals regardless of the thermistor reading. PCB faults are harder to diagnose because the symptoms overlap with several other causes. A technician typically rules out the simpler possibilities — filter, coil, gas, capacitor — before testing the PCB.

The capacitor provides the electrical boost the compressor needs on startup. When the capacitor weakens, it cannot deliver enough starting torque. The compressor attempts to start, draws high current, and the overload protector trips it off. After cooling down, the compressor tries again. This produces a distinctive pattern: a buzz or click from the outdoor unit, followed by silence, repeated every few minutes.

A capacitor test takes less than a minute with a multimeter. If the measured capacitance is below the rated value on the capacitor label, replacement is straightforward and inexpensive. This is one of the most cost-effective fixes for short-cycling — and one that is sometimes missed when the technician jumps straight to more expensive diagnoses.

Most short-cycling causes are not emergencies — a dirty filter or weak capacitor can wait a few days for a service visit. The exceptions are a burning smell during cycling, a breaker that trips on each restart, or visible sparking at the outdoor unit. Any of these warrants switching the unit off at the isolator until a technician inspects it.

For non-urgent cases, note the pattern before the visit. How long does the unit run before stopping? How long is the pause? Do you hear any sounds from the outdoor unit during the cycle? Does the indoor unit blow cold air during the brief run time? These observations help the technician narrow the cause faster.