Most air conditioners that run without cooling are choking on restricted airflow. A clogged filter, dirty coils or a frozen evaporator account for the majority of cases, and all three are things you can address. If those are clear, the likely cause is low refrigerant — which means a leak, and that is technician territory.
Start with airflow
An air conditioner cools by moving warm indoor air across a cold evaporator coil. Restrict that airflow and two things happen: less heat is removed, so the room stays warm; and the coil gets colder than intended, until moisture on it freezes into a block of ice that restricts airflow further. It is a loop that gets worse the longer it runs.
This is why a dirty filter — the cheapest, simplest part of the system — is behind so many “the AC has stopped working” calls.
Common causes
| Cause | How likely | Typical sign |
|---|---|---|
| Clogged air filter | Very common | Weak airflow at the vents; visible dust mat on the filter |
| Frozen evaporator coil | Very common | Ice on the indoor coil or pipework; airflow drops to almost nothing |
| Dirty condenser coil outside | Common | Outdoor unit caked in dust, grass clippings or leaves |
| Thermostat set or wired wrongly | Common | Fan runs constantly; set to FAN instead of COOL |
| Low refrigerant from a leak | Common | Gradual loss of cooling over weeks; hissing; ice at the line |
| Failed capacitor | Occasional | Outdoor fan hums but does not start |
| Failed compressor | Less common | Outdoor unit runs but nothing gets cold at all |
Working through it
Switch the system off at the thermostat and at the isolator or breaker before cleaning coils or removing panels. Outdoor condenser fans can start without warning, and capacitors inside the unit hold a dangerous charge even after the power is off — do not open sealed electrical compartments.
- Check the thermostat settings
Confirm it is set to COOL rather than FAN, that the target temperature is genuinely below room temperature, and that the batteries are good. A thermostat set to FAN will circulate air all day without cooling any of it.
- Replace or wash the filter
Take the filter out and hold it up to the light. If you cannot see through it, it is the problem. Replace disposable filters; wash reusable ones and let them dry completely before refitting. Do this monthly during heavy use.
- Clear the outdoor unit
Cut back plants to leave at least 60 cm of clear space all round, and remove leaves and debris from the fins. Rinse the fins gently from the inside outwards with a hose — never a pressure washer, which will flatten them.
- Check the vents and returns
Make sure furniture, rugs and curtains are not blocking supply or return vents. Closing off too many rooms also starves the system of return air.
- Look at the refrigerant lines
The larger insulated line should be cool and slightly sweating when running normally. Ice anywhere on the line, or a line that is not cold at all, points to an airflow or refrigerant problem.
- Check the condensate drain
A blocked condensate line can trip a float switch and shut the system down, or cause water to back up. Clear it if you can access it safely.
If the coil has frozen
Ice on the coil must be cleared before anything else can be judged, and it must be cleared gently:
- Switch the cooling off but leave the fan running. This thaws the coil with circulating air.
- Allow one to three hours. Do not chip, scrape or hit the ice — the coil and its fins damage very easily, and puncturing it turns a cheap problem into an expensive one.
- Put towels down; a fully iced coil releases a surprising amount of water.
- Once clear, fit a clean filter and run it again while you watch. If it freezes a second time with clean filters and clear vents, the cause is almost certainly low refrigerant.
An air conditioner does not consume refrigerant. If the charge is low, there is a leak, and adding more without finding and repairing that leak simply vents it again. Handling refrigerants requires certification in most countries, and releasing them is an offence in many.
When to call an HVAC technician
Call a technician if the coil freezes repeatedly with a clean filter, if you suspect a refrigerant leak, if the outdoor unit hums without starting, if the breaker trips when the system starts, or if the compressor runs continuously with no cooling at all. Anything involving the sealed refrigerant circuit or the unit’s electrical compartment belongs with a qualified engineer.
