Air Conditioner Not Cooling? 9 Causes, What You Can Fix, and When to Call a Technician

First, a two-minute test

Set the system to cool at 20 degrees, fan on high, and wait ten minutes. Hold a thermometer, or your hand, at a supply vent and compare with the air at the return grille. A healthy refrigerated system delivers supply air roughly 10 to 14 degrees colder than the return air. If the vents are cold but the room stays warm, you have an airflow, sizing or heat-load problem. If the vents are barely cooler than the room, you have a refrigeration or mechanical problem.

The 9 most common reasons an air conditioner stops cooling

1. Wrong mode or setpoint

More common than anyone admits. Dry mode, fan mode and auto mode all blow air without necessarily cooling. Check the controller shows a snowflake or cool symbol and the setpoint is at least three degrees below the current room temperature. On ducted systems, also check the zone you are standing in is actually open.

2. Dirty filters

A clogged filter starves the indoor coil of air. Cooling output drops, the coil can ice over, and the unit can trip on a protection fault. Wash the filters and let them dry before refitting. In Perth summers, filters need cleaning every two to four weeks in a busy household.

3. Blocked or dirty outdoor unit

The outdoor unit rejects the heat pulled from the house. If its coil is packed with dust, grass or cobwebs, or it is hemmed in by a fence or bins, it cannot dump heat and the system loses capacity, most obviously on the hottest days. Clear at least 500 mm around it and gently hose the fins from the inside out. A chemical coil clean by a technician restores heavily fouled coils.

4. Outdoor fan not running

If the outdoor unit hums or the compressor runs but the fan does not spin, the fan capacitor has usually failed. It is a small, cheap part, and one of the most common heatwave failures in Perth because capacitors degrade with heat. Switch the system off; a compressor running with no condenser fan overheats quickly. This is a licensed-electrical repair.

5. Low refrigerant from a leak

Symptoms are gradual loss of cooling over weeks, ice on the small copper pipe or the indoor coil, a hissing sound and the system running non-stop. Refrigerant is a sealed charge; it does not get used up, so low refrigerant always means a leak. A technician with an ARC licence will pressure test, locate the leak, repair it and then recharge to the manufacturer's specification. Regassing without repairing the leak is a waste of money and, under Australian refrigerant handling rules, poor practice.

6. Frozen evaporator coil

Ice on the indoor coil or the pipework is caused by low airflow, low refrigerant or running in cool mode when it is cold outside. Turn cooling off and run fan only for an hour or two to thaw it. If it refreezes with clean filters, the cause is refrigerant or a fan fault and needs a technician.

7. Blocked condensate drain

Many units have a float switch that stops cooling when the drain pan fills. Water dripping from the indoor unit or pooling near the outdoor unit points to a blocked drain, usually from dust, mould or an insect nest. A technician flushes the drain and treats the pan. Ignored, this leads to ceiling and wall damage.

8. Failed compressor, contactor or control board

If the indoor fan runs but the outdoor unit is completely silent, the contactor, control board or compressor may have failed, or the outdoor unit has lost power. Check the isolator switch next to the outdoor unit and the breaker in the switchboard. If they are on and the outdoor unit is dead, stop there and book a technician.

9. The system is undersized or the heat load has changed

If the vents are cold but a west-facing room never gets below 28 on a 40 degree afternoon, the unit may simply be too small for the load, or the load has grown: new glazing, removed shade trees, a darker roof, more people. This is a design issue, not a fault. Shading, insulation, a larger unit or a second unit are the options.

Quick diagnosis by symptom

Why Perth systems stop cooling on the hottest days

Perth has two conditions that push a marginal system over the edge. The first is the dry easterly: a run of 38 to 42 degree days where the outdoor unit is trying to reject heat into air hotter than its design point, so a condenser that is even slightly dirty, or a charge that is even slightly low, shows up as a room that will not get below 28. The second is the coastal salt air west of the freeway, which corrodes condenser fins and copper and turns a five-year-old unit into one that loses capacity every summer.

• If the system copes until about 35 degrees and then loses ground, book a condenser clean and a charge check before the next heatwave. That is the most common Perth call-out we attend, and the cheapest to fix in advance.
• Coastal suburbs from Fremantle to Joondalup should have condenser coils cleaned twice a year and inspected for fin corrosion. A coil coating at install time pays for itself here.
• Units on north or west walls in full afternoon sun run hotter than the same unit in shade. A simple shade screen that does not block airflow can recover several percent of capacity on a 40 degree day.
• In the eastern suburbs and hills, dust from summer easterlies packs condenser coils faster than near the coast, so check them after every dust event.

When to call a technician

• Any ice on the pipes or coil that comes back after thawing.
• The outdoor fan not running, or the outdoor unit silent while the indoor unit works.
• Water leaking indoors.
• A burning smell, tripping breaker or a unit that stops and restarts every few minutes.
• Supply air less than 8 degrees cooler than return air with clean filters.
• Any suspected refrigerant issue. Refrigerant handling requires an ARC licence in Australia.