An air conditioner that runs but doesn’t cool is maddening, usually because it picks the hottest week of the year to do it. The good news: plenty of “not cooling” complaints turn out to be something you can put right yourself in ten minutes, for free. Work through the checks below in order — we’ll flag exactly where the DIY stops.
Start with the settings
It sounds patronising, but it catches out plenty of people, especially with an unfamiliar remote or after a power cut. Check the unit is in cool mode — usually a snowflake symbol — not fan, dry, heat or auto. In fan mode it will happily blow room-temperature air all day. Then check the set temperature is a few degrees below the actual room temperature — set it to 20–21°C as a test, put the fan speed on high, and give it ten minutes. While you’re there, try fresh batteries in the remote; a weak remote can send garbled or partial commands.
Clean the filters
This is the most common cause of weak cooling we see. The filters inside the indoor unit catch dust constantly, and when they clog, airflow drops — the air feels tepid rather than cold, and in bad cases the coil behind them can freeze over. Lift the front panel of the indoor unit (it usually hinges up), slide the mesh filters out, and either vacuum them or rinse them under the tap. Let them dry fully before refitting. In heavy summer use it’s worth doing this every few weeks. It’s free, it’s safe, and it fixes a remarkable number of “broken” systems.
Look at the outdoor unit
The box outside is where the heat from your rooms actually gets dumped, and it needs to breathe. Check nothing is smothering it: leaves, overgrown plants, a fence panel leaned against it, bins pushed up close. Give it a clear gap — roughly half a metre — on all sides. Clear any debris by hand with the system switched off. While it’s running, look and listen — the big fan should be spinning when the indoor unit calls for cooling. If the indoor unit is running but the outdoor unit is silent, make a note of that; it’s useful information for an engineer, and often points to an electrical fault. Don’t open the casing.
Check the power and the breaker
Many installations have a separate isolator switch or breaker for the outdoor unit — sometimes on the wall beside it, sometimes back at the consumer unit. If a breaker has tripped, the indoor unit can seem alive while the outdoor half is dead. Reset it once. If it trips again, stop — a breaker that keeps tripping is protecting you from a real electrical fault, and forcing it back on can make things worse. (If your electrics are older or marginal, our guide on whether your home’s electrics can handle air conditioning is worth a read.)
If you see ice, thaw it first
Ice on the copper pipes or on the coil inside the indoor unit is a symptom, not the disease. Switch off cooling, run the unit in fan-only mode for an hour or so to thaw it, and put a towel down for the drips. If the filters were filthy, clean them — restricted airflow is the most common cause of icing. If the filters are clean and the ice comes back, that usually points to low refrigerant, which is an engineer’s job.
A quick symptom guide
| Symptom | Most likely cause | Who fixes it |
|---|---|---|
| Blows air, but never cold | Wrong mode or set point; failing that, a refrigerant leak | You first, then an engineer |
| Weak, tepid airflow | Clogged filters | You |
| Indoor runs, outdoor unit silent | Tripped breaker or a failed capacitor | You (one reset), then an engineer |
| Ice on pipes or coil | Poor airflow or low refrigerant | You thaw it; an engineer finds the cause |
| Cools briefly, then gives up | Blocked outdoor unit or a faulty sensor | Check clearance, then an engineer |
| Keeps tripping the electrics | Electrical or compressor fault | Engineer only |
The faults that need an engineer
Low refrigerant. A split system is sealed — refrigerant doesn’t get “used up”. If it’s low, it has leaked, and regassing without finding the leak means paying for the same visit again next summer. A good engineer will find and fix the leak first. Working on the refrigerant circuit legally requires F-Gas certification, so this is never a DIY job.
A failed capacitor. A common fault, especially on older units: the outdoor fan or compressor hums but won’t start. It’s typically one of the cheaper repairs, but capacitors hold a stored electrical charge even when the power is off, so leave it alone.
A failed compressor. The worst case. On an older system it’s worth weighing the repair quote against replacement — newer units heat as well as cool, and our air conditioning cost guide covers what a new install tends to cost.
Sensors and control boards. If the unit behaves erratically — cooling in bursts, ignoring the set temperature, flashing error codes — a sensor or board is often to blame. Note down any error code on the display; it can save an engineer real diagnostic time.
Where to stop
The honest line: cleaning filters, clearing debris from the outdoor unit, one breaker reset, changing remote batteries, and thawing ice in fan mode are all safely yours. Anything involving opening either unit, touching wiring, or the refrigerant circuit is not — because it’s dangerous, and because working on refrigerant without F-Gas certification is against the law. You won’t save money by having a go; you’ll usually turn a small repair into a bigger one.
If it does need a professional
If you’ve worked through the checks and the system still isn’t cooling, it’s engineer time. Describe the symptoms and anything you’ve ruled out — it genuinely helps. Coolhouse can match you with one vetted local engineer for air conditioning repairs, rather than passing your details to half a dozen firms who’ll all ring at once.