An air conditioner does not manufacture cold air; it removes heat from the room and rejects it outside. So “not cooling” always means the same thing physically: heat is not leaving the room fast enough. There are only a handful of reasons why, and they are easy to narrow down before anyone opens the unit.
The five-minute self check
- Hold a thermometer at the vent, then in the middle of the room. The difference should be 8–12°C.
- Pull the filters out. If light does not pass through them, that alone can explain weak cooling.
- Look at the outdoor unit: is the fan spinning, and are the fins grey with dust?
- Touch the two copper pipes at the outdoor unit — one should be cold and sweating, one warm. Ice on either is a fault.
- Listen: does the compressor start and stop every few minutes? That is an electrical or pressure problem, not a thermostat setting.
The nine causes, ranked
| Cause | Tell-tale sign | Who fixes it |
|---|---|---|
| Dust-blocked outdoor coil | Grey fins, weak hot air blast | Technician (wash) |
| Blocked filters | Weak airflow, ice on indoor coil | You |
| Low refrigerant / leak | Ice on copper line, small vent difference | Technician |
| Failed capacitor | Hums but the compressor will not start | Technician |
| Dirty blower wheel | Weak airflow, musty smell | Technician |
| Thermostat or sensor fault | Runs constantly or stops early | Technician |
| Compressor failure | Trips the breaker, no cooling at all | Technician |
| Unit too small for the room | Cools slightly, never reaches set point | Add capacity |
| Doors, windows, sun exposure | Cools at night, fails in the afternoon | You (insulation, curtains) |
How often we find each cause
One thing not to do
Setting the thermostat to 16°C does not cool the room faster. The unit removes heat at a fixed rate; the setting only decides when it stops. All a lower setting achieves is a compressor that never switches off, which raises the bill and shortens its life.
Frequently asked questions
Why does my AC cool at night but not in the afternoon?
Because capacity is marginal. At night the heat load is lower so the unit keeps up; at 45°C it cannot. That points to a dirty condenser, a low charge, or a unit too small for the room.
Is ice on the pipe a good sign?
No, it is always a fault. Ice means either low refrigerant or restricted airflow, and running the unit iced up can damage the compressor.
Can I clean the outdoor unit myself?
You can rinse loose dust with a gentle hose after switching the power off at the isolator. Avoid pressure washers — bent fins reduce cooling permanently.
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