Why your aircon isn't cooling: 8 things to check before calling a technician

It's 3 PM on a 34-degree Saturday. The split system's been running for two hours and the lounge room is still 28 degrees. You can hear the outdoor unit humming away, but the air coming out of the vents feels like a tired sigh.

By the end of this article, you'll know the eight most common reasons an aircon stops cooling — and which ones you can fix yourself in under 20 minutes.

Check the filter first (seriously, it's always the filter)

Pull the front cover off the indoor unit and slide out the filter. Hold it up to the light. If you can't see through it clearly, that's your problem.

A clogged filter cuts airflow by 40% or more. The system runs, the compressor works, but barely any cold air reaches the room. Clean it under a tap with warm water and a bit of dishwashing liquid. Let it dry completely before reinstalling — a damp filter grows mould.

In Sydney, especially near the coast, filters clog faster. Salt air, dust from building sites, eucalyptus pollen in spring. Every 4 to 6 weeks during summer if you're running it daily. More often if you have pets.

Did one in Lane Cove last March — three-bed weatherboard, ducted system from the late 90s. Meter draw actually went UP when they closed the back rooms. Took us an hour to figure out the dampers were wired backwards.

Check the thermostat setting

Sounds obvious, but someone changed it. Kids, cleaners, a guest who was cold at 2 AM.

Walk over to the controller. Is it set to COOL? Is the target temperature at least 3 degrees below the current room temp? If it's set to 24 and the room is 25, the system won't run hard.

Also check if it's in AUTO mode instead of COOL. AUTO switches between heating and cooling based on the set point, and in shoulder seasons (April, October) it can get confused. Force it to COOL.

If the screen is blank or unresponsive, the batteries might be flat. Replace them. If it's hardwired and still dead, that's an electrical fault — call a sparkie.

Inspect the outdoor unit for blockages

Go outside. Look at the condenser unit (the big metal box with the fan). Is it surrounded by garden clippings, leaves, or a hedge that's grown into the fins?

The condenser needs airflow on all sides. If the fins are clogged with debris, the system can't dump heat. It'll run constantly and produce weak cooling.

Turn the system off at the isolator switch (usually on the wall near the unit). Use a hose on low pressure to rinse the fins from the inside out. Don't use a pressure washer — you'll bend the fins. If they're already bent, you can buy a fin comb from Bunnings for about $18 and straighten them yourself, but honestly it's fiddly.

Clear at least 50 cm of space around the unit. Trim back plants. Move the bin if it's blocking airflow.

Check for ice buildup on the indoor coil

Open the front cover of the indoor unit again. Look at the evaporator coil (the metal grid behind the filter). Is there frost or ice on it?

Ice means one of two things: airflow is too low (back to the filter), or refrigerant is too low (see next section). Sometimes both.

If there's ice, turn the system off and let it thaw completely — takes 2 to 4 hours. Run the fan only (no cooling) to speed it up. Once it's thawed, clean the filter and try again. If ice comes back within a day, you've got a refrigerant leak or a failing fan motor. Book a tech.

Look for signs of refrigerant leak

Refrigerant doesn't evaporate. It doesn't "run out". If your system is low, there's a leak somewhere.

Signs:

Ice on the indoor coil or the copper pipes running to the outdoor unit.
Weak airflow even with a spotless filter.
The outdoor unit runs constantly but the room stays warm.
A hissing sound near the indoor unit (rare, but if you hear it, turn the system off immediately).

You cannot top up refrigerant yourself. It's illegal in Australia without an ARCtick licence, and doing it wrong can wreck the compressor. Call a licensed HVAC tech. Expect $280 to $450 for a leak test and re-gas, depending on the system size and refrigerant type.

Most refrigerant leaks happen at the joints where the copper pipes connect to the indoor or outdoor unit. Vibration loosens them over time. A good tech will pressure-test the system before re-gassing — if they just top it up without finding the leak, you'll be back in six months.

Check the circuit breaker and isolator switch

Walk to your switchboard. Find the breaker labelled AIR CON or HVAC. Is it tripped (lever in the middle position or fully off)? Flip it back on.

If it trips again immediately, there's a short circuit somewhere. Don't keep resetting it. Call an electrician.

Also check the isolator switch near the outdoor unit. It's usually a metal box on the wall with a red switch. Someone might have turned it off by accident (gardeners do this constantly).

Verify the compressor is actually running

Stand next to the outdoor unit while the system is set to COOL and the thermostat is calling for cooling. You should hear the compressor kick in — a low hum or vibration, plus the fan spinning.

If the fan is running but there's no hum and no vibration, the compressor isn't starting. Could be a failed capacitor (cheap fix, $120 to $180), a seized compressor (expensive, $1,800 to $3,200 depending on system size), or a faulty control board.

This is where you stop and call someone. Compressor diagnosis requires a multimeter and experience.

Check for closed or blocked vents (ducted systems only)

If you have ducted aircon, walk through the house and check every vent. Are they all open? Is furniture blocking any of them?

Closing too many vents increases static pressure in the ducts. The system works harder, moves less air, and cooling suffers. Most ducted systems are designed to run with at least 70% of vents open.

Also check the return air grille (usually in a hallway ceiling). If it's blocked by storage boxes or covered by a rug, the system can't pull enough air through. Weak return air = weak cooling.

The Wenest take

In the homes we work in across Sydney, the version of this that actually fails is the outdoor unit being slowly strangled by garden growth. Homeowners don't look at it for months. By the time they notice weak cooling, the condenser fins are packed solid with leaves and the compressor is overheating every afternoon.

The second most common? Filter neglect. We've pulled filters out of split systems in the Eastern Suburbs that haven't been cleaned in two years. The airflow was so weak the evaporator coil was icing up every night.

If you'd rather not be the one remembering to clean filters every six weeks or crawling around the side of the house with a hose, that's literally what Wenest members get — scheduled maintenance before things stop working.

Frequently asked questions

Why is my split system running but not cooling the room?

Most often it's a dirty filter restricting airflow, or the outdoor unit is blocked by debris. Check both before calling a technician. If the compressor isn't running at all (the outdoor unit is silent), it could be a thermostat issue or electrical fault requiring a licensed HVAC tech.

How do I know if my aircon is low on refrigerant?

Signs include ice buildup on the indoor coil or copper lines, weak airflow even with a clean filter, and the outdoor unit running constantly without cooling. Refrigerant doesn't 'run out' — a leak is always the cause. Only a licensed technician can legally top up refrigerant in Australia.

Can I run my aircon if it's not cooling properly?

Not for long. Running an aircon with a refrigerant leak or blocked condenser can damage the compressor — an expensive repair. If you've checked the filter and thermostat and it's still not cooling, turn it off and book a service call.

How often should I clean my aircon filter in Sydney?

Every 4 to 6 weeks during summer if you're running it daily. More often if you have pets or live near the beach (salt air clogs filters faster). A clogged filter is the single most common cause of weak cooling we see in Sydney homes.

If none of the above fixes it, or you'd rather hand the whole thing off, Wenest coordinates HVAC maintenance for members across Sydney. We schedule it before you notice the problem.