A unit that blows air but doesn’t cool is one of the most common faults we’re called about, and it usually shows up on the hottest day of the year. Sometimes the fix is a setting or a dirty filter. Sometimes it’s a refrigerant leak or a failed component that needs a qualified engineer.
This guide takes you through the checks that are safe to do yourself, the warning signs that mean you should switch off, and the faults an engineer will test for. It also explains why “just regas it” is rarely the right answer.
Checks you can do yourself
- Mode. Make sure the remote shows cooling, usually a snowflake, not heat, fan, dry or auto with a high setting.
- Set temperature. It must be below the room temperature. Check any timer, sleep or eco mode isn’t limiting output.
- Remote batteries. A weak remote may change the display without reaching the unit.
- Filters. Lift the front panel and look. Clogged filters choke the airflow. Our filter cleaning guide shows how to wash them.
- The outdoor unit. Is its fan turning? Is anything blocking it, such as leaves, a cover, bins or plants grown up around it?
- Doors and windows. A unit sized for a closed room won’t cope with open patio doors.
Is it a fault, or just a very hot day?
Every system is sized for a design condition. On an unusually hot day, in a room with lots of glass in full sun, a correctly sized unit may run flat out and still only get the room a few degrees cooler. That isn’t necessarily a fault. If the air coming out of the indoor unit feels clearly cold, the system is probably working. If it feels barely cooler than the room, something is likely wrong. Our heatwave tips explain how to help a system that’s working hard.
Signs you should switch off
Switch off at the remote and the isolator and call an engineer if you see:
- ice on the indoor unit, the pipework or the outdoor unit in summer;
- water dripping from the indoor unit;
- a burning smell, buzzing or tripping electrics;
- grinding, rattling or loud clicking;
- a flashing light or error code that comes back after a restart.
Take a photo of any code. Our guides to Samsung, Mitsubishi Electric and Daikin error codes explain what many of them mean. Ice has its own guide: ice on an air conditioning unit.
Want it done properly? AirCon Doctor installs, repairs and services air conditioning and heat pumps across London and the South East, from our base in East Grinstead. We’re an F-Gas certified company and NICEIC approved. Get a fixed-price quote.
Low refrigerant means a leak
Refrigerant isn’t used up like fuel. If the charge is low, it has leaked out somewhere. Typical symptoms are a slow decline in cooling over weeks, ice on the pipework and the system running constantly. Flare joints, damaged pipes and corroded coils are common culprits.
Topping up without finding the leak only buys time, and the gas escapes again. The right fix is to find the leak, repair it, pressure test, evacuate and recharge to the correct weight. That work must be done by a qualified engineer working for an F-Gas certified company, as GOV.UK’s certification guidance explains. Our guide to air conditioning regas covers the process.
Other faults an engineer looks for
- Dirty coils. A coated indoor coil or a blocked outdoor coil can’t move heat properly.
- Outdoor fan motor. If the outdoor fan doesn’t run, the system overheats and cuts back or stops.
- Sensors. A failed temperature sensor can make the unit think the room is already cool.
- Compressor or inverter board. Electronic faults can stop the compressor while the indoor fan keeps blowing.
- Expansion valve. A stuck valve starves or floods the coil.
- Condensate pump alarm. On many installations a tripped pump safety switch stops cooling to prevent a leak.
What a repair visit involves
A proper diagnosis starts with the history and any stored fault codes, then electrical checks, then refrigerant pressures and temperatures with the system running. Only then do we say what the fault is and what it will cost to fix. Our repair diagnosis starts from £75 + VAT, and you get a fixed price for the repair before any parts are fitted.
See our air conditioning repair page, or for urgent faults, our 24/7 emergency repair line.
Repair or replace?
For most modern systems, a repair is the sensible choice. For older units, especially those on R22 or R410A, it’s worth weighing up the age, the refrigerant and the cost of the repair. R22 can no longer be used to top up systems, and R410A is getting scarcer and dearer as supply is phased down. Our guide to repairing or replacing R410A and R22 systems sets out the choice.
Quick answers
Why is my air conditioning blowing warm air?
Check the mode, set temperature, filters and outdoor unit first. If those are fine, a refrigerant leak, a failed fan, sensor or compressor fault is likely, and an engineer needs to test it.
Can I regas my air conditioning myself?
No. Handling refrigerant requires F-Gas qualifications, and work must be done by a certified company. A top-up without fixing the leak won’t last anyway.
Why does my unit cool for a while and then stop?
Common causes are a blocked filter or coil, an outdoor unit overheating, a condensate pump alarm or an electronic fault. Note any error code and call us.
How much does it cost to diagnose a fault?
Our repair diagnosis starts from £75 + VAT. You get a fixed price for the repair before work goes ahead.
Related services
- Air conditioning repair
- Emergency air conditioning repair
- Air conditioning regas
- Air conditioning servicing
More advice
- Air Conditioning Regas: What It Is and When You Need It
- Ice on Your Air Conditioning Unit: What It Means and What to Do
- Why Is My Air Conditioning Leaking Water? Causes and Fixes
- What Happens During an Air Conditioning Emergency Call-Out
- R410A and R22 Air Conditioning: Repair or Replace?
- All advice
This guide is general information, checked against the sources linked on 30 October 2026. It isn’t legal, tax or financial advice. Rules, grants and product ranges change, so check the current position before you rely on it.