Air Conditioning Not Cooling? 7 Causes, From Free Fixes to Failed Parts

Work through these in order — it's roughly the sequence a service engineer follows, and the first three cost nothing.

1. Mode and setpoint (free)

Obvious but genuinely common: the unit is in fan, dry or heat mode, or the setpoint is above room temperature. Look for the snowflake icon, set it 4–5°C below room temperature, and give it ten minutes.

2. Dirty filters (free)

The most common real cause. Clogged filters strangle airflow — the unit runs but moves little air, and what comes out barely feels cold. Open the front panel, slide the filters out, wash in warm water, dry fully, refit. Do this every 4–6 weeks in heavy use.

3. Blocked outdoor unit (free)

The outdoor coil must reject heat. Leaves, fluff, cottonwood and garden debris on the coil — or anything stacked against it — kills capacity. Switch off, clear debris, gently rinse the coil fins (no pressure washer).

4. Remote/sensor faults (cheap)

Flat remote batteries, or a faulty return-air thermistor telling the unit the room is already cold. If the unit behaves erratically — short bursts, wrong readings — a thermistor is a £15–£40 part.

5. Refrigerant leak (engineer)

Symptoms: ice on the indoor coil or pipework, hissing, gradually worsening cooling over weeks, the unit running constantly. Refrigerant doesn't get "used up" — if it's low, it leaked, and regassing without fixing the leak is throwing money away. F-Gas qualified engineer only.

6. Failed fan motor (part + engineer)

If the indoor unit is silent with no airflow, or the outdoor fan isn't spinning, you're looking at a fan motor. Common failure on 8–15 year old units — usually £80–£250 for the part. We stock fan motors for the major brands.

7. Failed PCB or compressor (the big two)

PCB: unit dead, unresponsive, or flashing an error code. Boards run £150–£500 depending on brand and are usually worth fitting. Compressor: the heart of the system — on an older unit, a failed compressor usually tips the repair-or-replace maths toward a new system, especially with modern units being 30–40% more efficient.

Repair or replace?

Rule of thumb: if the repair costs more than a third of a new installed system and the unit is over 10 years old, replace. New air-to-air systems may also qualify for the £2,500 grant if you're replacing electric heating.

Need the part? Send us your unit's model number (on the sticker, side of the indoor or outdoor unit) and we'll identify the exact filter, thermistor, fan motor or PCB. Spares lookup →