Air Conditioner Not Cooling? Common Causes and What to Check

An air conditioner not cooling is usually down to one of six things, and they run in a rough order of likelihood: a dirty filter, a blocked outdoor unit, the wrong mode or thermostat setting, a tripped breaker, low refrigerant, or dirty coils. The first four are safe for a homeowner to check in a few minutes, and they solve a large share of call-outs before a technician is even needed. The last two, refrigerant and coils, are always a licensed job. This guide walks through each cause in that order so you can rule out the easy ones first.

Perth summers do not forgive a struggling system, and an air conditioner not cooling on a scorching day is miserable, so it pays to know what to look at and, just as importantly, what to leave alone.

Why your air conditioner is not cooling: the six usual causes

Before you assume the worst, it helps to know that an air conditioner not cooling well is far more often a maintenance issue than a dead compressor. Here are the six causes to work through, from the most common to the least.

1. A dirty or clogged filter

This is the number one reason for an air conditioner not cooling properly. The filter sits behind the front panel of the indoor unit, and it slowly fills with dust until airflow drops to a trickle. Less air across the coil means less cooling in the room, and a badly blocked filter can even ice the coil over and stop cooling entirely. Filters are designed to be cleaned by the homeowner: slide the panel open, lift the filters out, rinse them under the tap, let them dry fully and slot them back in. Do this every few weeks over summer and you remove the single most common cause of weak cooling.

2. A blocked or dirty outdoor unit

The outdoor unit dumps your home’s heat outside, and it can only do that if air moves freely through it. Leaves, grass clippings, cobwebs and garden growth pressed against the fins choke it, and a smothered outdoor unit is a classic cause of an air conditioner not cooling on a hot day. Switch the system off at the wall, then clear away any debris, pot plants or rubbish within about half a metre of the unit and gently hose loose dirt off the outside of the fins. Keep the area around it clear as a habit. Do not open the unit or bend the fins yourself.

3. The wrong mode or thermostat setting

It sounds obvious, but a surprising number of “air conditioner not working” complaints come down to the remote. A reverse cycle unit set to “fan” or “auto” may push air without cooling it, and a thermostat set higher than the room temperature will never kick the cooling in. An air conditioner blowing warm air is very often just this: set the remote firmly to “cool” (the snowflake symbol), drop the target temperature a few degrees below the current room temperature, and make sure any timer or “econo” mode is not holding it back. This is the fastest fix of all, and it costs nothing.

4. A tripped breaker or power issue

If the unit is completely dead, or the outdoor unit is silent while the indoor head runs, the circuit may have tripped. This is a common reason an air conditioner is not working at all. Find your switchboard and look for a breaker in the off or middle position, then switch it fully off and back on once. If it holds, you are back in business. If it trips again straight away, stop there: a breaker that keeps tripping is signalling an electrical fault, and that is a job for a licensed technician, not a second reset.

5. Low refrigerant

Refrigerant is the gas that actually carries heat out of your home, and a system does not use it up in normal running. If it is low, there is a leak, and that leak is what leaves an air conditioner not cold even with a clean filter and a clear outdoor unit. You cannot check or top up refrigerant yourself, and you should not try: handling it is licensed work by law. If the easy checks come up clean and you still have an air conditioner not cooling, low refrigerant is a likely suspect and the point to book a technician.

6. Dirty coils or a failing part

Over years of service the indoor and outdoor coils build up a grime layer that insulates them and quietly strangles performance, and a worn fan motor or capacitor can do the same. These are internal jobs that need the unit opened, cleaned or tested with proper tools, so they sit firmly on the technician’s side of the line. Regular servicing is what keeps coils clean and catches a tired part before it leaves you with an air conditioner not cooling in the middle of a heatwave.

What you can safely check yourself

The safe DIY list is short and worth memorising. You can clean or replace the filters, change the mode and temperature on the remote, reset a tripped breaker once, and clear leaves and debris away from the outdoor unit. Between them, those four checks resolve a big proportion of air conditioning not cooling cases, and none of them require opening the sealed parts of the machine. Always switch the system off at the wall before you touch the outdoor unit.

When an air conditioner not cooling needs a technician

Everything past the easy checks belongs to a licensed technician. Refrigerant work, any electrical repair, coil cleaning inside the unit and compressor faults are not homeowner jobs, both because they need specialist tools and because refrigerant, gas and electrical work are regulated for safety. Call a professional if the breaker trips repeatedly, if you have an air conditioner blowing warm air after you have confirmed the settings and cleaned the filter, if you hear grinding or hissing, or if the unit ices up. Trying to force a fix beyond the simple checks usually turns a small repair into a larger one.

To understand why a reverse cycle system heats and cools from the same box, and what that means when it stops doing one of them, our explainer on how reverse cycle air conditioning works is a useful read. And if warm air is only half the story and you are also seeing water where it should not be, our guide to an air conditioner leaking water covers that separately.

Get your air conditioner cooling again

If you have worked through the safe checks and you still have an air conditioner not cooling, it is time for a professional. Our licensed local team services and repairs air conditioners across Joondalup and the wider Perth area, north and south of the river, and we will diagnose the fault properly rather than guess. Browse our air conditioning Perth hub, see our split system air conditioners range, or call the team on (08) 6200 9718 for honest local advice.

Frequently asked questions

For the bigger picture on choosing and sizing a system, compare ducted air conditioning versus split systems, or talk to our Perth team about a service for your home.