AC Not Cooling? 6 Common Causes and What to Check
When it’s 30°C outside and your AC is running but the house won’t cool, it’s miserable. Before you call for service, here are the six most common causes — starting with the ones you can check yourself.
1. A dirty air filter
This is the number-one cause of weak cooling. A clogged filter chokes airflow, so even a healthy AC can’t move enough cool air. Check your filter — if it’s grey and packed, replace it and give the system an hour.
2. Thermostat settings
Make sure it’s set to Cool, the fan is on Auto (not On, which runs the fan even when the system isn’t cooling), and the target is below room temperature. Dead thermostat batteries can also cause odd behaviour.
3. A blocked or dirty outdoor unit
The outdoor condenser needs airflow to dump heat. If it’s buried in leaves, grass clippings, or shrubs, cooling suffers. Gently clear a couple of feet around it and hose off obvious debris (power off first).
4. Blocked vents and returns
Closed or furniture-blocked supply vents and return grilles reduce airflow through the whole system. Walk the house and make sure they’re open and clear.
5. Frozen evaporator coil
If you see ice on the indoor unit or the refrigerant lines, the coil has frozen — usually from restricted airflow or low refrigerant. Turn the AC off and let it thaw (running it frozen can damage the compressor), then check your filter. If it refreezes, it’s time for a pro.
6. Low refrigerant
If your AC is low on refrigerant, it can’t cool properly — and low refrigerant usually means a leak. This isn’t a DIY fix: handling refrigerant requires a licensed technician, and simply “topping it up” without finding the leak is a temporary band-aid.
When to call a pro
If you’ve checked the filter, thermostat, outdoor unit, and vents and it still won’t cool — or you’re seeing ice, hearing odd noises, or suspect a refrigerant leak — get a technician on it. Tell us what’s happening and we’ll match you with available local pros, including same-day options during heat waves.