Why is my air conditioner not cooling? Causes and fixes

If your air conditioner is running but not cooling, the cause is usually simple and free to fix. Nine times out of ten it is the wrong mode or temperature setting, a dirty filter choking the airflow, or something blocking the unit, rather than a broken machine. Start with the settings and the filter, then work through the airflow, and only reach for the phone if the unit is icing up, has quietly lost its cooling over weeks, or the room is simply too hot for the unit you have.

The one thing to know up front: a sealed home split does not "use up" refrigerant the way a car does, so it almost never needs a routine "re-gas". If a split has genuinely lost cooling power because the gas is low, there is a leak, and by law only an F-Gas registered engineer can find and fix it. Everything else on this page you can safely check yourself. Here is the quick diagnosis, then each cause in the order worth trying.

Quick diagnosis: symptom, likely cause, fix

Find the symptom that matches yours, then read the full section below it.

Check the mode and temperature first

This sounds obvious, and it is the most common reason a unit "blows but isn't cold". Air conditioners have several modes, and only one of them actually cools:

• Cool mode runs the compressor and the fan together to lower the temperature. This is the one you want on a hot day.

• Fan mode turns the compressor off and only moves air around. It makes no change to the temperature at all, so the room stays as warm as it was.

• Dry (dehumidify) mode is built to pull moisture out of the air, not to cool. It does not blow much cold air, so it will not bring down a hot room.

Set the unit to cool, then set the target temperature a few degrees below the current room temperature. If the room is 25°C and the unit is set to 26°C, it has no reason to cool. Around 20 to 22°C is a sensible target for comfort without overworking the unit. On a reverse-cycle unit (one that heats as well as cools), also check it has not been left in heating mode from the winter, and replace the remote's batteries if the display is blank.

Clean the filter

A clogged filter is the single most common airflow problem. Dust builds up on the mesh, starves the unit of air, and cooling drops off, sometimes to the point where the coil freezes.

Cleaning it is a two-minute job. Switch the unit off, slide out the filter (behind the front flap on a wall unit, or a panel on a portable), and either vacuum it gently or rinse it in lukewarm water with a little mild soap. Let it dry completely before you put it back, because a damp filter breeds smells. Do this every one to three months while the unit is in use, and more often for a portable running hard in a heatwave. If cooling jumps back up afterwards, the filter was your problem. Our air conditioning maintenance guide walks through filter cleaning and the rest of the routine upkeep in more detail.

Clear the airflow, inside and out

Air conditioners cool by moving heat from inside to outside, so they need a clear path for air at both ends.

Indoors, make sure nothing is blocking the unit. Furniture, curtains or a shelf too close to a wall unit, or clutter around a portable, will choke it. Give a portable at least 50cm of clear space around its air intakes.

Outdoors, the condenser (the box on the wall or ground outside) throws out the heat your unit removes. If it is buried in leaves, blocked by a fence or bin, or clogged with debris, that heat has nowhere to go and cooling suffers. With the unit switched off, clear anything within about half a metre and gently brush debris off the fins. Use a soft brush or a low-pressure hose only, never a pressure washer, which bends the delicate fins.

Portable air conditioner not cooling? Read this

Portables are the most common home unit in the UK, and most "not cooling" complaints come down to how they are vented rather than a fault. Work through these in order.

Sort out the exhaust hose. The hose carries hot air out of the window, and if that heat cannot escape, it leaks straight back into the room. Keep the hose as short and as straight as you can. A hose that is kinked, coiled, or bent more than about 30 degrees restricts the airflow badly, so route it in a clean run to the window and raise the unit on a low table if that helps keep the hose flat.

Seal the window properly. The foam window kit that comes in the box rarely seals completely. Any gap lets hot outside air pour back in as fast as the unit cools. Tape or weatherstrip around the bracket and any leftover gaps; it takes ten minutes and makes a real difference.

Understand single-hose versus dual-hose. Most cheap portables have a single hose. As they blow warm air out, they draw replacement air from the rest of the house, and on a very hot day some of that pulled-in air is warm, which limits how cold the room gets. A dual-hose unit takes its air from outside separately, holds the room's pressure better, and copes far better in real heat. If your single-hose unit simply cannot keep up in a large or sunny room, that design limit may be why. Our best portable air conditioners guide covers which types suit which rooms.

Check the water tank. Many modern portables are self-evaporating: they recycle most of the condensate and push it out through the exhaust, so they rarely need emptying. In a humid spell or after long running, the tank can still fill and the unit will shut off automatically to protect itself. Empty the tank, or fit a continuous drain hose if the model allows, and stand the unit level so the tray does not overflow.

Be realistic about the room. Portable cooling ratings are often optimistic, so a unit sold for a big room may only really handle a smaller one. If yours runs flat out and never quite gets there, it may simply be undersized. See what size air conditioning do I need to check, and remember that a conservatory or a glass-heavy room can need roughly double the cooling of a normal room.

There's ice on the unit

Ice on the coils or pipework stops a unit cooling, which feels backwards but makes sense: restricted airflow (or low refrigerant) lets the coil get so cold that moisture freezes on it, and a block of ice cannot transfer heat.

To fix it, switch the unit off and let the ice melt away completely, which can take an hour or two. While it thaws, clean the filter and clear any airflow blockage, because that is usually the cause. Then restart it. If it cools normally, airflow was the problem. If it ices up again, stop: repeated freezing usually points to low refrigerant, and that is a job for an engineer, not more defrosting.

It keeps shutting off, or the power is dead

If the unit switches itself off after a while, that is often normal: it has reached the target temperature and will restart when the room warms up again. Set the target lower if you want it to keep running. On a portable, a shut-off can also be the full-tank cut-out described above.

If the unit is completely dead, or a split's outdoor unit never runs, check for a tripped breaker in your consumer unit and reset it once. For a portable, plug it straight into a wall socket rather than an extension lead, try a different socket, and check the plug fuse. If a breaker trips again as soon as you reset it, do not keep resetting it, that is an electrical fault to have checked by a qualified electrician.

Is it just too hot, or the wrong size unit?

Sometimes nothing is broken. On a very hot day the unit has to work far harder, because the gap between the outdoor temperature and your target is so wide. The UK has hit 40.3°C, and the UK Health Security Agency notes that even fans stop helping much above 35°C, so on the hottest afternoons even a healthy, correctly sized unit may only take the edge off a sunny, poorly shaded room rather than make it cold.

If the unit runs constantly and never reaches target even on an ordinary warm day, it is probably undersized for the space. Closing curtains or blinds on sun-facing windows, cooling the room earlier in the day, and cutting other heat sources all help. If it is genuinely too small, no amount of troubleshooting will fix it, and a larger or additional unit is the honest answer.

Low refrigerant: why your split does not need a "re-gas"

This is where the car-aircon myth causes real confusion. A car's air con is expected to lose gas slowly and get topped up. A home split is a sealed system that should never lose refrigerant in normal use. So if a split's cooling has faded over weeks, the coil keeps icing, or you hear hissing, the charge is probably low, and that means a leak somewhere in the circuit.

The fix is not a routine top-up. The leak has to be found and repaired first, then the system recharged, and by law that work can only be done by an F-Gas registered engineer (usually REFCOM registered). Handling refrigerant on a split without certification is a criminal offence, not just a warranty risk, partly because modern refrigerants like R32, and the propane (R290) in many portables, are flammable. If you suspect low refrigerant, stop troubleshooting and get a certified engineer to look at it.

Sealed portables and monoblocs are the exception: they are factory-sealed with nothing to top up, so if one has truly lost its gas, that is a warranty or replacement question, not a DIY one.

What you can fix yourself, and what needs a pro

As a rule of thumb, anything to do with settings, filters, airflow, hoses, the water tank and resetting a tripped breaker is safe, free DIY. Anything to do with the refrigerant circuit, the compressor, repeated coil icing or repeated electrical tripping needs a professional.

A fault that needs a professional does not have to cost a fortune. As a rough guide in August 2026, a service and clean runs about £70 to £150, finding and repairing a refrigerant leak commonly £100 to £300 or more, and larger repairs £200 to £600, with a compressor replacement running up to around £800. On an older portable, weigh a big repair against simply replacing it.

Costs are rough UK bands correct as of August 2026 and vary by unit, fault and installer. Always get a quote before committing. Electricity is capped at 26.11p/kWh (July to September 2026); the cap changes quarterly, and from 1 October 2026 VAT on domestic electricity is cut from 5% to 0%.

Fixing airflow problems promptly also keeps running costs down, because a dirty, struggling unit burns more electricity for less cooling. Our guide to whether air conditioners are expensive to run explains the numbers, and a yearly professional service heads off most of these faults before a heatwave.