Air Conditioning Not Cooling: Causes, Checks and Repair Costs
The fan is running, the remote says 18°C, and the room is still warm. Here are the causes in the order engineers find them, what is safe to check yourself, and what each repair costs in 2026.
The six causes, in the order engineers find them
Air conditioning stops cooling for surprisingly few reasons. Many "not cooling" call-outs are resolved by airflow or settings; the rest split between refrigerant, electrical parts and, rarely, the compressor.
1. Dirty filters or a blocked coil
The most common cause by a distance. Once the mesh filters clog, too little air passes over the cold coil, the unit cannot shed heat and may begin to ice up. Lift the front panel, slide the filters out, wash in lukewarm water, dry and refit. In a home running its air conditioning hard through a hot spell, once a month is about right; a wall unit in a dusty room or with pets around clogs faster than that. If it is over a year since a service, the coil behind the filters may be furred up too, and a blocked condensate drain can start the unit cutting out — both need a proper clean by an engineer rather than a rinse at the sink.
2. Wrong mode, set-point or timer
Check the remote is in cool (snowflake) rather than fan, dry or auto, that the set temperature is below room temperature, that eco or sleep mode is not capping output and that a timer has not put the unit into standby. Flat remote batteries cause more "breakdowns" than anyone admits. On a multi-split every head must be in the same mode — one in heat blocks the others from cooling.
3. Frozen evaporator coil
Weak airflow plus frost on the pipes or ice behind the filters means the coil has frozen. Switch off and let it thaw for a couple of hours — expect dripping. Restricted airflow is the benign cause; if it freezes again with clean filters, the charge is low and you are at cause four.
4. Low refrigerant — which always means a leak
Split systems are sealed and do not "use up" gas, so a low charge means refrigerant has escaped through a flare joint, corroded coil or damaged pipe. Symptoms: cooling that has faded over weeks, hissing, ice on the thin pipe, sometimes an error code. The usual escape points are the flare joints where the pipework meets each unit, a corroded outdoor coil, or pipe runs that have chafed against a wall. Most modern splits are charged with R32, which is mildly flammable (an A2L refrigerant), so a DIY top-up is not only illegal without certification — done wrong it is genuinely unsafe. Only an F-Gas certified engineer may trace and fix the leak, pressure-test and re-gas; our re-gas cost guide explains why a top-up alone is not an option.
5. Failed capacitor or fan motor
Go outside. If the outdoor fan is not turning, or the unit hums and clicks but never starts, a run capacitor or fan motor has usually failed — routine, same-day repairs where the bill is mostly the call-out.
6. Compressor
Least common, most serious. A compressor that will not start, trips the breaker or knocks loudly is a stop-and-think moment: on a unit over ten years old, part and labour often exceed half the cost of a new system. See repair or replace? first.
What you can check yourself — and what needs an F-Gas engineer
Anything behind the filter panel or inside the outdoor casing involves mains voltage, a pressurised refrigerant circuit or both. Clean, look and reset — never open, connect or top up.
| Check | Safe to do yourself? | Why |
|---|---|---|
| Clean or replace indoor filters | Yes | No tools, no electrics — just the front panel and a sink |
| Clear leaves, covers and stored items from the outdoor unit | Yes | The outdoor coil needs free airflow on all sides; keep 30 cm clear |
| Switch off at the isolator, wait ten minutes, restart | Yes | Clears many protection lock-outs and some error codes |
| Open the outdoor unit, test capacitors or wiring | No | Capacitors hold a charge after power is off; live terminals inside |
| Add refrigerant, open valves or fit gauges | No — illegal without F-Gas certification | Venting refrigerant is an offence and a top-up without a leak repair is wasted |
If a check on the left fixes it, book a service anyway — neglect brings the fault back.
Typical repair cost by cause (2026)
Call-outs typically run £80–£150 and include diagnosis plus minor work; parts are extra and refrigerant work is priced separately. Ranges are UK baselines — get quotes for your unit. Many engineers will credit the diagnostic call-out against the bill if you go ahead with the repair on the day, so it is worth asking when you book.
| Cause | Typical fix | Typical cost |
|---|---|---|
| Dirty filters / blocked coil | DIY filter wash, or engineer coil clean as part of a service | £0 DIY · £75–£150 service |
| Frozen evaporator (airflow cause) | Thaw, clean, check fan — call-out only | £80–£150 |
| Low refrigerant (leak) | Find and fix leak, pressure test, re-gas | £100–£300 re-gas + leak repair |
| Failed capacitor or outdoor fan motor | Replace on the day — call-out plus the part | £80–£150 + part |
| Compressor | Quote basis — compare with a new unit | Often uneconomic vs £1,500–£3,000 new |
Baselines: call-out £80–£150; re-gas £100–£300; single split installed £1,500–£3,000. Full breakdown in air conditioning repair costs.
When "not cooling" means it is time for a new unit
Three signals tip the balance. Age: past 10–15 years units lose efficiency and parts get scarce. Refrigerant: R22 cannot legally be re-gassed and R410A is being phased down under F-Gas quotas; today's units run on R32. The 50% rule: if the repair quote exceeds half the price of a new system — £1,500–£3,000 for a single wall-mounted split — buy a new, warrantied unit. Daikin and Mitsubishi Electric carry 5–7 year warranties through accredited installers. Filter, capacitor or fan: repair. Compressor, or a leak on an old R410A system: read repair or replace? and get both quotes.
Sources & methodology
- Fault order reflects the diagnostic sequence in manufacturer service manuals (Daikin, Mitsubishi Electric, Fujitsu, Panasonic, LG, Samsung).
- Refrigerant handling: GB F-Gas Regulation (retained Regulation (EU) 517/2014) — only certified persons may recover, charge or check for leaks; venting is an offence.
- Price baselines are 2026 UK figures used across this site: call-out £80–£150, re-gas £100–£300, service £75–£150, single split installed £1,500–£3,000.
FAQs
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