Why is my air conditioner not cooling?

An air conditioner that runs but does not cool is one of the most common calls we get, and the cause is usually one of six things. They are worth understanding in order, because two of them you can fix yourself in twenty minutes and the rest tell you what you should be paying for.

First: the two things to check yourself

1. The filters

Open the indoor unit and pull out the mesh filters. If they are grey with dust, that is very likely your answer. A blocked filter starves the evaporator coil of airflow, and with too little air passing over it the coil gets colder and colder until it ices over. Once there is ice on the coil, airflow drops further and cooling stops entirely.

Wash them in lukewarm water, let them dry completely, and refit them. If the unit has been running with ice on the coil, switch it to fan-only for an hour to melt it before you run cooling again.

2. The outdoor unit

Go and look at it. The fan should be spinning and it should be blowing warm air. If it is packed with dust, leaves or building debris, or if something has been stacked against it, it cannot dump heat outside — and an air conditioner works by moving heat outside, not by making cold. Clear at least half a metre of space around it.

Then: what a technician checks

3. Refrigerant charge — and the leak behind it

Low refrigerant is the cause everyone has heard of, and the one most often handled badly. Here is the part that matters: a sealed refrigeration system does not consume refrigerant. It is a closed loop. If the charge is low, refrigerant has escaped, which means there is a leak.

Anyone who tops up your gas without finding that leak has sold you a few months. The system will empty again, you will pay again, and in the meantime the compressor runs starved of the refrigerant that also carries its oil — which is how a gas leak turns into a dead compressor.

The correct sequence is: pressure test, find the leak, repair it, evacuate the system, then recharge to the manufacturer weight. It costs more on the day and far less over the life of the unit.

4. A fouled condenser coil

Different from a dirty outdoor unit that you can see. Dust and sand work between the aluminium fins and pack solid, and the coil stops rejecting heat even though it looks clean from the outside. Pressures climb, cooling drops, and the compressor works harder for less. It needs washing out from the inside so the debris is pushed back out of the fins rather than deeper into them.

5. A failing compressor or capacitor

If the outdoor fan spins but the compressor does not start, or it hums and trips out, the usual suspect is the start capacitor — an inexpensive part and a straightforward replacement. If the compressor itself has failed, the decision becomes economic: on a unit over about ten years old running an obsolete refrigerant, replacement is usually the honest recommendation.

6. Sensors and control board

A displaced or failed room-temperature sensor makes the unit believe the room is already at temperature, so it throttles back or stops. The room stays warm while the unit insists it is finished. Cheap to fix, easy to miss if nobody checks.

What about the remote?

Worth ruling out before anything else: confirm the unit is in cool mode rather than fan or dry, that the set temperature is genuinely below room temperature, and that fan speed is not stuck on minimum. It is not the usual cause, but it costs nothing to check and we have driven across the city for it more than once.

How we approach it

We diagnose before quoting. Electrical readings, running pressures, and temperature differential across the coil will normally identify the fault within the first half hour, and you get a fixed price before any work starts. If the unit is not worth repairing we will tell you that rather than sell you a repair that will not last.