When your air conditioner stops working, it’s not always easy to know what went wrong. Some issues are simple and can be fixed quickly. Others need a professional. This guide explains the most common reasons why your AC may have stopped working and what you can do in each case.
We’ll go from the easiest problems you can check and fix on your own, to the ones that require technical knowledge or tools.
1. The thermostat is set incorrectly or not working
Start by checking your thermostat. It controls how and when your air conditioner runs, so even a small setting error can stop the system from working.
What to check:
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Make sure it’s set to cooling mode and not heating or fan-only.
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Check that the set temperature is lower than the current room temperature.
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If your thermostat uses batteries, check if they need to be replaced.
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Confirm that the display is on and responding to changes.
In many cases, you can sort it yourself by adjusting the settings, changing the batteries, or doing a quick reset. If the screen is blank, a new set of batteries often brings it back to life. But if the thermostat still doesn’t respond, keeps turning off, or resets itself for no reason, there’s likely a wiring fault or internal issue. At that point, it’s best to have an electrician take a proper look.
2. Tripped circuit breaker or blown fuse
If your air conditioner suddenly stops working and won’t turn on at all, one of the first things to check is the power supply. A tripped breaker or a blown fuse can cut power to the entire system.
What to check:
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Go to your fuse box and look for the switch labelled for the air conditioner.
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If the breaker is in the middle or ‘off’ position, it has likely tripped.
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For older systems, check for a fuse near the outdoor unit or inside the disconnect box.
You can reset the breaker by switching it fully off and then back on. If it stays in place and the unit turns on, it was likely a one-time overload. The same applies if you’re replacing a fuse — just make sure the power is off first, and use the correct type.
But if the breaker trips again shortly after resetting, or the fuse blows again, there’s probably an underlying electrical fault. This could be caused by a short circuit, a damaged capacitor, or even a motor issue, and it’s not something you can or should handle on your own. A technician will need to find and fix the root cause safely.
3. Clogged air filter
A clogged air filter can block airflow through your system, causing it to shut down or blow warm air. It can also lead to other problems, such as frozen coils or overheating.
What to check:
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Locate your air filter — it’s usually behind the return air grille or inside the indoor unit.
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Take it out and hold it up to the light. If you can’t see through it, it’s dirty and needs changing.
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Check how long it’s been since the last replacement. Most filters require changing every 1–3 months.
You can replace the filter yourself. Just make sure you get the right size and type. Once the filter is in, turn the system back on and see if airflow and cooling return to normal.
If your air conditioner doesn’t improve after changing the filter, or if it shuts down again soon after starting, the issue may have gone further, such as ice forming on the coil or strain on the fan motor. In that case, it can be fixed only with the help of a specialist.
4. Blocked or closed vents
If your air conditioner is running, but the airflow feels weak or uneven across rooms, the vents might be blocked or closed.
What to check:
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Walk through each room and check that the vents are open.
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Make sure no furniture, curtains, or rugs are covering them.
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Look for signs of dust build-up or debris around the vent openings.
You can clear the area around the vents and open any that were closed. This helps restore proper airflow and allows the system to cool the space more evenly. Sometimes, people close vents in unused rooms to save energy, but this can put pressure on the system and lead to problems over time.
If you’ve checked all the vents and the airflow is still poor, there might be a blockage inside the ductwork or an issue with the blower fan. These are not easy to access without the right tools, so you’ll need to call for help to inspect and clean the system properly.
5. Dirty or blocked condenser unit
The outdoor condenser unit needs clear airflow to release heat from your home. If it’s clogged with dirt, leaves, or other debris, the system can overheat or stop cooling properly.
What to check:
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Go outside and look at the unit. Check if there’s anything blocking the airflow around the sides or top.
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Look between the metal fins — if they’re packed with dust or leaves, the unit needs cleaning.
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Make sure there’s at least a 30 cm (1 foot) gap around the unit on all sides.
You can clear debris by hand and use a garden hose to gently rinse the fins from the inside out. Make sure the power is off before you do this. Don’t use high pressure — it can bend the fins.
If the unit is still struggling after basic cleaning, the dirt may have built up deeper inside, or there could be damage to the fan or coil. In that case, a full service is needed to restore proper function.
6. Refrigerant leak or low refrigerant levels
If your air conditioner is running but only blowing warm air, or if you notice ice on the pipes or the indoor unit, the system may be low on refrigerant. This usually means there’s a leak.
What to check:
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Feel the air coming from the vents — is it warm or not as cool as usual?
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Look at the copper pipes near the indoor or outdoor unit. Ice build-up is a strong sign of low refrigerant.
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Listen for hissing or bubbling sounds near the refrigerant lines, which could indicate a leak.
Unfortunately, you can’t refill refrigerant yourself — handling it requires certification and proper tools. Even if it seems like a small issue, running the system with low refrigerant can damage the compressor. So if you suspect a leak, turn the unit off and contact the AC technician. They’ll need to locate the leak, repair it properly, and recharge the system to the correct level.
7. Frozen evaporator coil
The evaporator coil sits inside your indoor unit and is responsible for absorbing heat. If it freezes over, your air conditioner may blow warm air or stop working entirely.
What to check:
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Remove the front panel of the indoor unit if possible and look for ice on the coil or surrounding pipes.
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Check for reduced airflow from the vents — this often happens when the coil is frozen.
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Make sure the air filter is clean and the vents are open. Poor airflow is one of the main causes of freezing.
If you see ice, turn off the system and let it defrost completely. This can take a few hours. Once the ice has melted, check the filter and airflow, then turn the unit back on. In case it freezes again, there may be a deeper issue such as low refrigerant, a dirty coil, or a fan problem. So best of all, I recommend calling the AC specialist for a proper inspection.
8. Faulty capacitor or contactor
If your outdoor unit isn’t starting at all, or if it tries to start and then shuts off quickly, the problem might be a faulty capacitor or contactor. These components help start and run the compressor and fan motor.
What to check:
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Listen for clicking sounds from the outdoor unit — this can be a sign the contactor is trying to engage but failing.
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A loud humming followed by silence might point to a capacitor issue.
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Check if the fan in the outdoor unit starts spinning. If it doesn’t, or stops shortly after starting, one of these parts could be the cause.
You won’t be able to test or replace these parts safely without the right tools and experience. They carry high voltage even after the power is off, which makes them dangerous to handle without proper precautions. The best you can do in this case, is stop using the system and get help from the specialists. Keep in mind, that replacing capacitors and contactors is a common repair, but it needs to be done correctly to avoid further damage.
9. Motor or compressor failure
If your air conditioner isn’t turning on at all, or if it’s making buzzing noises without cooling, the motor or compressor could have failed. These are key components that drive the system, and when they fail, the AC usually stops working altogether.
What to check:
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Is the outdoor unit completely silent or making a loud buzzing noise?
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Does the fan try to start but never spin?
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Have you already ruled out the thermostat, capacitor, and circuit breaker?
There’s nothing you can do here without technical knowledge and the right equipment. Motors and compressors are sealed parts and often require full replacement, which, as you might guess, isn’t that cheap.
Final word
If you’ve gone through all the basic steps — checking the thermostat, power supply, filter, and airflow — and your air conditioner still isn’t working, there may be an internal fault that isn’t visible from the outside.
What to check:
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Review what you’ve already tried. Has the system shown any signs of life at all? Any strange sounds or smells?
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Make sure all switches and isolation points are turned on, including any switch near the indoor or outdoor unit.
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If the system has a reset button, try using it once — but only after switching off the power for a few minutes.
At this stage, the problem is likely something you won’t be able to confirm or fix without proper tools. It could be an electrical fault, a wiring issue, or a failed control board. Continuing to run or test the system may cause further damage. So your best option now is to book a full AC repair inspection.
If this is your case, then fill out the contact form and we will arrive within 24 hours. One of our technicians will test each part of the system to find the fault and recommend the most efficient way to get it running again.