Air conditioning not cooling: what to check

Start with the settings and airflow before assuming the system needs refrigerant.

The answer in 2 lines. Check that cooling mode is selected and the target temperature is below the room temperature. Look for blocked air inlets, dirty filters and obstructions around the outdoor unit. Follow your appliance instructions before removing any user-accessible filters. Persistent poor cooling, ice or an error code needs an engineer's assessment: none proves a refrigerant leak by itself. Stop using the system if it trips the electrics, smells burnt or leaks near electrical fittings.

The first checks

Check the controls: choose cooling rather than heating, fan-only or dehumidifying mode. Check the target temperature, timer and remote-control batteries.
Check the filters: switch off and isolate the appliance as its instructions require. Clean or replace only filters described as user-accessible.
Check airflow: move curtains and furniture away from indoor vents. From a safe position, look for leaves or stored objects obstructing the outdoor unit.

Stop: danger

Burning smell, crackling or repeated tripping: stop using the equipment. Isolate its supply only if safely accessible, and arrange a professional inspection.
Smoke or fire: leave, stay outside and call 999. Do not use water on an electrical fire or delay leaving to switch off the power.

Work through the simple causes

Check whether the fault affects every indoor unit or just one room. Note what the display shows before changing settings. A timer, an operating restriction or an error message may explain why the system is not responding. Use the instructions for your exact model rather than a generic sequence found online.

For a portable air conditioner, check that its exhaust hose and window arrangement match the instructions. Check any tank-full warning and the specified draining procedure. A portable unit and a fixed split system have different arrangements; do not apply one appliance's draining or reset instructions to another.

What the symptoms can tell you

Weak airflow: check accessible filters and blocked vents. If airflow stays weak, an internal fault or contamination needs assessment.
Air moves but does not cool: record the settings and any error code. Controls, airflow, operating conditions or a refrigeration fault could be involved.
Visible ice: stop cooling and arrange advice if it persists or returns. Do not chip it off or apply heat.
Poor performance during hot weather: reduce direct sunlight and unnecessary indoor heat. An engineer can assess capacity if the problem continues.

Blocked outdoor airflow can reduce cooling performance: the official air-conditioning inspection guide explains this under its assessment of refrigeration equipment.

What not to do

Buying a DIY refrigerant refill or accepting a recharge without investigating why performance has fallen.
Opening electrical covers, reaching through fan guards or working on refrigerant pipework.
Repeatedly resetting a tripping circuit breaker or washing an operating outdoor unit with a hose.

Arrange the right repair

Send the engineer the model details, error message and a description of the checks you have completed. Ask for a written explanation of the proposed diagnosis, repair and charges before authorising work.

For equipment containing F gas, check both the individual's qualifications and the business's certification. Use GOV.UK's working-with-F-gas guidance to understand the separate requirements. Ask the contractor for evidence relevant to your equipment. Do not assume a general electrical qualification covers refrigerant work.

If you rent in England

Report the fault in writing, with photographs and the error code, before commissioning repairs yourself. Check the inventory and tenancy agreement for the supplied cooling equipment. Responsibility can depend on whether the system also provides heating, what was supplied and how the fault arose.

Read GOV.UK's renting repairs guidance to check the general responsibilities and what to do when repairs are not addressed. Wales, Scotland and Northern Ireland have different housing rules; this section concerns England.

Good to know

Follow the equipment's maintenance instructions. Energy inspections and refrigerant leak checks are separate requirements; neither provides a universal domestic cleaning schedule.
An error code helps an engineer investigate, but it does not necessarily identify the failed component.
Keep photographs, service records and the repair report together, particularly if the fault returns.

Illustrative example: in Nottingham, Alex finds that the bedroom unit blows weakly. After isolating it according to the instructions, Alex cleans the washable filters and lets them dry before refitting them. Airflow improves, but cooling does not. Alex records the display message and asks a qualified engineer to investigate rather than ordering refrigerant.

A made-up story, for illustration.

Bobby

Sources, verification, confidence

Verified on 5 November 2026, for England. Confidence: high for F-gas qualification requirements and fire precautions; medium for troubleshooting, which depends on the model and requires diagnosis if basic checks do not resolve the fault. No universal servicing interval or repair price is asserted.