How to Fix an Air Conditioner: Common Problems and What to Do About Them

How to Fix an Air Conditioner: Start With the Basics

Few things are more frustrating than an air conditioner that stops working on a 38-degree Australian summer afternoon. Before you call a technician and wait days for a booking, it is worth knowing that a large number of common AC faults have straightforward fixes you can handle yourself. This guide covers how to fix an air conditioner by walking through the four main troubleshooting areas: identifying the problem, running through DIY checks, knowing when to call a pro, and deciding whether repair or replacement makes more sense.

The Most Common Air Conditioner Problems (And What Causes Them)

Most air conditioner faults fall into a handful of categories. Understanding what is likely causing the problem is the first step toward fixing it quickly, without spending money on a service call you may not need.

Here are the five most common AC problems Australian homeowners run into:

1. Unit not cooling or heating - Dirty filters, low refrigerant, wrong mode selected, or a unit that is undersized for the room.
2. Aircon not turning on - Tripped circuit breaker, blown fuse, flat remote batteries, or the unit locked in a fault protection mode.
3. Water leaking from the indoor unit - A blocked condensate drain line is the most common cause. A frozen evaporator coil that has thawed can also produce a sudden leak.
4. Strange noises or smells - Rattling usually points to a loose panel or debris in the outdoor unit. A musty smell almost always means mould or bacteria growing on a dirty filter or evaporator coil.
5. Unit runs but never reaches the set temperature - This typically means the system is undersized, the room has poor insulation, the filter is partially blocked, or the refrigerant charge is low.

Why Is My Aircon Not Cooling?

An aircon that runs but blows warm air is one of the most common complaints, and the cause is usually one of four things. Work through these in order before calling a technician.

1. Check the mode setting. Make sure the unit is set to 'Cool' mode, not 'Fan' or 'Dry'. It sounds obvious, but a wrong mode selection is responsible for a surprising number of service calls.
2. Inspect the air filter. A clogged filter restricts airflow across the evaporator coil, dramatically reducing cooling output. Remove the filter and hold it up to the light. If you cannot see through it, clean or replace it.
3. Check the outdoor unit. Make sure the condenser unit outside is not blocked by vegetation, debris or direct sun with no clearance. The outdoor unit needs free airflow to reject heat.
4. Consider whether the unit is the right size. A 2.5kW split system will struggle to cool a 30-square-metre room on a 40-degree day. If the unit has always underperformed, sizing may be the root cause.
5. Low refrigerant. If the above checks come back clear, low refrigerant caused by a leak is likely. This requires a licensed refrigeration technician to diagnose and repair. You cannot top up refrigerant yourself.

For a deeper look, see our guide on air conditioner not cooling.

Why Won't My Air Conditioner Turn On?

If your aircon is completely unresponsive, start with the simplest explanations before assuming the worst.

1. Check the circuit breaker. A tripped breaker is the single most common reason an aircon will not turn on. Go to your switchboard and look for any breaker that has tripped to the middle or 'off' position. Reset it and try the unit again.
2. Replace the remote batteries. A flat remote will make the unit appear dead when it is actually fine. Swap in fresh batteries before anything else.
3. Check for a blown fuse. Some older systems and outdoor disconnect boxes have fuses. A blown fuse will cut power to the unit entirely.
4. Look for a fault or protection mode. Many modern inverter systems will lock out and display an error code if they detect a fault. Check the indoor unit's display panel for any flashing lights or codes, then cross-reference the error code in your owner's manual.

If the breaker keeps tripping after you reset it, do not keep resetting it. A repeatedly tripping breaker points to an electrical fault that needs professional attention. Read our guide on air conditioner tripping the circuit breaker to understand what is likely causing it.

DIY Fixes You Can Safely Do Yourself

Before you book a technician, work through these seven checks. They cost nothing, take less than 30 minutes, and resolve the majority of common air conditioner faults. These steps apply to most split system brands including Daikin, Mitsubishi Electric, Fujitsu and Samsung.

1. Check and reset the circuit breaker. Go to your switchboard and look for any breaker in the tripped or 'off' position. Flip it fully off, then back on. A tripped breaker is one of the most common reasons an aircon appears completely dead, and resetting it takes 10 seconds.
2. Replace the remote control batteries. A flat remote will make a perfectly functional unit seem unresponsive. Swap in a fresh set of AA or AAA batteries before assuming anything is wrong with the unit itself.
3. Clean or replace the air filter. A clogged filter is the single most common cause of reduced cooling and heating performance. Remove the filter from the indoor unit, rinse it under warm water, let it dry fully, and refit it. For a full walkthrough, see our guide on how to clean your AC filter. Most manufacturers recommend cleaning the filter every two to four weeks during heavy use.
4. Check the thermostat and mode settings on the remote. Confirm the unit is set to the correct mode ('Cool' in summer, 'Heat' in winter) and that the set temperature is actually lower than the room temperature for cooling, or higher for heating. An incorrect mode setting is behind a surprising number of service calls.
5. Clear debris from around the outdoor unit. Walk outside and check that the condenser unit has at least 30cm of clear space on all sides. Remove any leaves, grass clippings or garden debris that have built up around or inside the unit. A blocked outdoor unit cannot reject heat efficiently, which causes the system to underperform or shut down on protection mode.
6. Check that the condensate drain is not blocked. Find the drain line running from your indoor unit, usually a small pipe exiting through the wall or ceiling. If it is dripping slowly or not at all during cooling, it may be partially blocked. A blocked drain causes water to back up and leak from the indoor unit. Flushing the drain line with a small amount of water can clear minor blockages.
7. Reset the unit by powering it off at the wall for 30 seconds. Switch the unit off at the wall isolator or power point, wait 30 seconds, then switch it back on. This clears minor software faults and resets the unit's protection mode, which can lock the system out after a power fluctuation or temporary fault.

If the unit still does not behave normally after working through all seven steps, the fault is likely beyond DIY territory. That is where the next section comes in.

When to Call a Licensed Technician (And When a New Unit Makes More Sense)

Some air conditioner faults are genuinely beyond what a homeowner can or should attempt to fix. Knowing where that line sits saves you time, money and the risk of making a problem worse. If any of the following apply to your unit, stop troubleshooting and call a licensed professional.

• Refrigerant leak or low gas charge. If your unit is running but not cooling and all the DIY checks come back clear, a refrigerant leak is the likely culprit. In Australia, handling refrigerants requires an ARC (Australian Refrigeration Council) licence. Attempting to regas a system yourself is illegal under the Ozone Protection and Synthetic Greenhouse Gas Management Act, and the fines are substantial. This is a job for a licensed refrigeration mechanic.
• Faulty capacitor or fan motor. If the outdoor unit hums but the fan does not spin, or the system struggles to start, a failed capacitor or motor is the likely cause. These involve live electrical components and are not safe to handle without proper training.
• Compressor failure. A failed compressor is the most expensive repair in any air conditioning system. Symptoms include the unit running continuously without cooling, loud clanking from the outdoor unit, or the system tripping its breaker repeatedly.
• Electrical faults inside the unit. Burning smells, scorch marks on the indoor unit, or a breaker that keeps tripping after reset all point to an internal electrical fault. Turn the unit off at the wall and do not use it until a licensed electrician has inspected it.
• Persistent error codes. If your unit is displaying a fault code that does not clear after a full power reset, the code is pointing to a specific component failure. Cross-reference the code in your owner's manual, but if it keeps returning, a technician needs to diagnose it properly.

Repair vs Replace: How to Decide

Once you have a repair quote in hand, the decision to fix or replace comes down to a simple rule of thumb. If the repair cost is more than 50% of what a comparable new unit would cost, replacement almost always wins on value. Factor in the age of the unit as well. A 12-year-old system that needs a $700 compressor repair is a poor investment when a brand-new unit with a full manufacturer's warranty costs $1,000 to $1,400.

Modern inverter split systems are also significantly more energy-efficient than units from 10 or more years ago. A new unit with a high star rating can meaningfully reduce your electricity bills over time, which means the cost of replacement is partially offset by lower running costs year after year.

To give you a sense of what quality replacement units cost in 2026: the Daikin 3.5kW Cora Inverter Split System (FTXV35WVMA) is $1,256, making it a solid entry-level option from one of the most trusted brands in Australia. Step up to the Daikin 3.5kW ALIRA X (FTXM35YVMA) at $1,412 and you get built-in WiFi for smartphone control as a notable upgrade. For smaller rooms, the Fujitsu 2.5kW Lifestyle Range Inverter Split System (ASTG09KMTC) comes in at $1,014 and is a compact, reliable option that suits bedrooms and studies well.

If a technician is quoting you more than half of those figures to repair an ageing unit, the numbers are telling you something worth listening to.

Frequently Asked Questions

What is the most common AC repair?

The most common AC repair is clearing a blocked or dirty air filter, followed by refrigerant top-ups (regassing) and fixing a tripped circuit breaker. Dirty filters are responsible for a large proportion of reduced-performance complaints and are something most homeowners can fix themselves in minutes. Cleaning your filter every four to six weeks during periods of heavy use will prevent the majority of these issues before they start.

Is AC good for BP patients?

Air conditioning can be beneficial for people with high blood pressure because it helps maintain a stable, comfortable temperature, reducing the cardiovascular strain that comes with extreme heat. However, very cold settings or sudden temperature changes between indoors and outdoors may cause blood vessels to constrict. People with blood pressure concerns should keep their aircon set to a moderate temperature, around 22 to 24 degrees Celsius, and avoid drastic temperature swings between rooms or between inside and outside.

Can AC cause sinus issues?

Yes, air conditioning can contribute to sinus irritation, particularly if the unit's filter is dirty and circulating dust, mould spores or allergens through the room. Running the aircon at very low temperatures can also dry out nasal passages over time. Regular filter cleaning and keeping the temperature at a comfortable level rather than extremely cold can significantly reduce sinus-related discomfort for most people.

Is AC harmful for bronchitis?

Air conditioning is not inherently harmful for people with bronchitis, but a poorly maintained unit can worsen symptoms by circulating dust, mould and other airborne irritants. Cold, dry air from an aircon set too low can also irritate already-sensitive airways. Keeping the filter clean, maintaining a moderate temperature and ensuring good ventilation are the best ways to use air conditioning safely if you have bronchitis.

Ready for a Reliable Replacement? Browse Our Range

Most common air conditioner problems have a straightforward fix, and working through the DIY steps in this guide will resolve the majority of faults without a service call. But if your unit is ageing, the repair quotes are stacking up, or it has simply never performed the way it should, a new split system is often the smarter move.

Modern inverter systems are quieter, more energy-efficient and far better value than units from a decade ago. Whether you are replacing a tired old unit or setting up a room that has never had cooling, browse our full range of split system air conditioners from Daikin, Fujitsu, Mitsubishi Electric and more. All units come with Australia-wide delivery and competitive pricing direct from our online warehouse.