If your air conditioner runs but is not cooling, check that the mode is set to cool (the snowflake), the filters are clean, and the outdoor unit is running and clear of leaves and clutter. If the outdoor fan isn’t spinning, the pipes are icing up, or the air stays warm after those checks, the unit needs a technician. The usual causes then are a failed fan or capacitor, a refrigerant leak, or a compressor fault.
Work through the checks first. They fix a large share of “not cooling” problems without a call-out.
Five checks first
- Mode and set temperature. Make sure the remote shows cool, not fan, dry or auto, and that the set temperature is below the room temperature. A setting of 23 to 25 degrees is realistic on a hot day.
- Filters. Pull the filters out. If they are grey or matted, vacuum and rinse them, let them dry, and refit. A blocked filter is a common cause of poor cooling.
- Outdoor unit. With the system cooling, the outdoor fan should spin and you should feel warm air leaving it. Clear anything within a hand span of the coil.
- Doors, windows and sun. A unit sized for one room will struggle with an open plan area, open doors or full afternoon sun on west-facing glass.
- Give it time. After a hot day, walls and furniture hold heat. Allow 20 to 30 minutes before judging.
Causes and what each one needs
| Cause | What you’ll notice | DIY or technician |
|---|---|---|
| Wrong mode or set temperature | Fan runs, air is room temperature | DIY |
| Dirty filters | Weak airflow, sometimes ice on the indoor coil | DIY |
| Blocked or dirty outdoor coil | Outdoor unit runs hot; cooling fades on hot afternoons | DIY clear-around; technician to clean the coil |
| Outdoor fan or capacitor failed | Outdoor unit hums but the fan doesn’t spin | Technician |
| Refrigerant leak (low gas) | Cooling gets weaker over weeks; ice on pipes; hissing | Technician (ARCtick-licensed) |
| Frozen indoor coil | Ice on the pipes or coil, water dripping later | Switch to fan mode to thaw; find the cause |
| Compressor or control fault | Outdoor fan runs but no cooling; error code | Technician |
| Unit too small for the space | Runs flat out all day and never reaches the set temperature | Technician to assess sizing |
Is it low on gas?
Refrigerant does not get used up. If a system is short of gas, it has a leak. Topping it up without finding the leak only buys time. Signs include cooling that fades over weeks, ice on the thin copper pipe at the outdoor unit, and a hissing or bubbling sound.
In Australia only an ARCtick-licensed technician can handle refrigerant. A proper repair finds and fixes the leak, then evacuates and recharges the system to the manufacturer’s specified amount.
Not cooling on very hot days
Every system has a rated capacity. On a 40 degree day with the sun on the glass, a correctly sized unit may only hold the room a few degrees below outside, and an undersized one may not manage that. If the unit cools well on mild days and struggles only in extreme heat, the cause may be sizing or a dirty outdoor coil rather than a fault.
Brand guides
- Mitsubishi Electric not cooling properly
- Daikin not cooling
- Samsung not working: common problems
- AC fan not working
When to call a technician
Call if the outdoor fan isn’t turning, there is ice on the pipes after the filters are clean, a breaker trips, or cooling keeps getting weaker. If the system is old and the diagnosis points to the compressor, read whether it is worth repairing an old AC before deciding.
Book a technician online or start from our troubleshooting hub for other symptoms.
In Australia, refrigerant work must be done by a technician who holds an ARC licence. You can read more about licensing on the ARCtick website.
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Reviewed by AC repair and service specialist on 22 October 2026. How we write our articles