Aircon Not Cooling? 7 Common Causes (Gauteng Guide)

This is one of our most common call-outs across Gauteng's northern suburbs — large split and multi-unit installations in homes and estates around Sandton, Bryanston, Fourways, Waterfall and Bedfordview see heavy summer use and tend to surface these faults first. Everything below applies whether it's one wall unit in a Midrand apartment or a multi-zone system in a Waterfall estate home.

1. Dirty or blocked filter

This is the single most common cause of an aircon that seems weaker every week rather than suddenly dead. Dust builds up on the indoor unit's filter, airflow drops, and the unit works harder for less cooling. It's also the one fault you can check and fix yourself in five minutes — most indoor units have a filter that slides out from the front panel without tools.

2. Low refrigerant gas

Aircon gas doesn't get "used up" in normal operation — if the level is low, there's a leak somewhere in the line or coil. Symptoms: the unit runs, air comes out, but it's barely cooler than room temperature, and ice sometimes forms on the indoor unit's pipes. This needs a technician with gauges to find the leak, repair it, and recharge to the correct pressure — topping up gas without fixing the leak just means doing it again in a few months.

3. Blocked or dirty outdoor unit

The outdoor unit needs airflow through its coil to dump heat. Leaves, dust, or being boxed in too tightly by walls or plants all restrict this, and the unit struggles to cool even though nothing is actually broken. Worth a visual check — if the coil fins are visibly caked in dust or debris, that's likely a contributing factor.

4. Frozen indoor coil

If you can see ice or frost on the indoor unit, don't keep running it — switch to fan-only mode to let it thaw before it damages the compressor. A frozen coil is usually a downstream symptom of low gas or restricted airflow (see points 1 and 2), not a fault on its own.

5. Faulty compressor

The compressor is the heart of the system, and when it's failing you'll usually hear it — a humming sound with no cooling, or the outdoor unit not starting at all. Compressor faults are the most expensive to fix and, on an older unit, sometimes cheaper to replace the whole unit than repair.

6. Thermostat or control board fault

Sometimes the aircon is cooling fine, but the sensor reading room temperature is wrong, so it cycles off too early or runs on the wrong setting. This shows up as inconsistent cooling rather than a total loss of cold air.

7. Undersized unit for the room

Not a fault at all — if a unit was installed a size too small for the room (or the room's been extended since installation), it will genuinely struggle to cool on very hot days no matter how well it's serviced. Worth ruling out if the unit has always struggled, rather than having gotten worse recently.

What you can check yourself first

• Clean or replace the filter (usually slides out the front of the indoor unit)
• Check the outdoor unit isn't blocked by leaves, boxes, or overgrown plants
• Check the remote's mode isn't accidentally set to fan-only or dry mode instead of cool
• Check the circuit breaker hasn't tripped

If none of that resolves it, it's worth booking a diagnostic call-out rather than guessing further — a technician can pressure-test the gas and check the compressor in one visit. See our aircon repair Sandton and aircon repair Randburg pages for area-specific service, or the homepage for full coverage across Gauteng.