Aircon Not Cooling

Running but the room never gets cool — here is what is actually wrong.

An air conditioner that runs happily but never cools is the most common call we take in a Sydney summer. The good news: about a third of these are fixable by the owner in ten minutes. The rest come down to a handful of mechanical and refrigerant faults that a licensed technician can identify quickly. This guide walks through both, in the order worth checking.

Quick answer

If your aircon runs but does not cool, check the mode is set to cool, clean the indoor filters, and make sure the outdoor unit is running and unobstructed. If those are fine, the likely causes are low refrigerant from a leak, a dirty indoor or outdoor coil, a failed compressor capacitor, or an undersized system. Professional diagnosis costs $99–$150 and typical repairs run $250–$900.

Key takeaways

Dirty filters and blocked outdoor units cause a large share of weak cooling
Low refrigerant always means a leak — never just a top-up
If the outdoor unit never starts, suspect the capacitor, contactor or board
Ice on the indoor coil means restricted airflow or low gas: turn it off and call
A system that only struggles on 38°C days may simply be undersized

Start with the five-minute checks

Remote set to Cool (snowflake), not Fan, Dry or Auto, with setpoint at least 3°C below room temperature
Indoor filters removed, rinsed and fully dried — do this every few months in summer
Outdoor unit running, with at least 30cm of clear space and no leaves against the coil
Isolator switch on and circuit breaker not tripped
Doors, windows and curtains closed on the sunny side of the house

The mechanical causes we find most often

Once the basics are ruled out, cooling failure comes down to heat not being moved out of the house. That happens for four main reasons.

Low refrigerant from a leak: cools weakly, may ice up, pressures read low
Dirty coils: the outdoor coil cannot reject heat, so the room barely drops
Failed capacitor or contactor: compressor hums or never starts, fan may still spin
Failed reversing valve: system stuck in heating or neutral, blows ambient air

Why it cools some days and not others

Systems sized to the bare minimum cope fine at 28°C and fall behind at 38°C. If your unit holds temperature on mild days but drifts up in a heatwave, capacity — not a fault — is the issue. Adding shading, sealing draughts and upgrading insulation helps, but the real fix is correct capacity next time you replace.

When to switch it off and call

Ice on the indoor coil or pipework — running it iced can destroy the compressor
Burning smell, tripping breaker, or loud grinding
Water pouring rather than dripping from the indoor unit
Hissing near the pipework, which suggests a refrigerant leak

What it costs to fix an aircon that is not cooling

Sydney pricing for the repairs behind poor or absent cooling.

Item	Typical price
Diagnostic and fault report — Credited against the repair	$99 – $150
Deep clean of indoor and outdoor coils	$250 – $450
Capacitor or contactor replacement	$250 – $450
Leak detection, repair and regas	$400 – $900
Outdoor fan motor replacement	$450 – $850
Reversing valve replacement	$700 – $1,400
Compressor replacement	$1,400 – $2,600

All prices include GST. You approve a fixed repair price after diagnosis, before work begins.

Frequently asked questions

Why is my aircon blowing cool air but the room stays hot?

Usually the system is moving air but not enough heat — a partly blocked coil, low refrigerant or a system too small for the space. Poor insulation and unshaded west-facing glass also let heat in faster than the unit removes it.

Can I just add gas myself?

No. Handling refrigerant without an ARC licence is illegal in Australia, and adding gas without fixing the leak wastes your money and vents greenhouse gas.

How long should it take to cool a room?

A correctly sized system should drop a closed room by several degrees within 15 to 20 minutes and hold setpoint after that. Much longer than 30 minutes with no progress points to a fault or a sizing problem.

Does cleaning filters really make a difference?

A significant one. Clogged filters choke airflow across the coil, which cuts cooling capacity, freezes coils and drives up power use. It is the single most valuable thing an owner can do.

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