Air Conditioning Not Working? Start with the Symptom
Use safe checks to rule out controls and airflow, recognise problems that need an engineer, and avoid turning a small fault into water or electrical damage.
Air conditioning fault finder
Stop the system if you see smoke or sparking, smell burning, find damaged wiring, hear severe mechanical impact, see water near electrical parts, or experience repeated breaker tripping. Move people away where necessary and use the emergency repair guide.
Air conditioner or heat pump not heating
If your heat pump is not heating, start with mode and temperature. Select the sun or heating symbol, set the target above the measured room temperature and allow several minutes for compressor protection and warm-up. Many indoor fans deliberately pause so they do not blow cold air while the coil warms. In cold or damp weather, a normal defrost cycle can also pause indoor heat and create steam or water outside for several minutes.
Check user-cleanable filters and obvious airflow without opening fixed covers. On a multi-split, every connected indoor unit may need a compatible mode; one controller asking for cooling can conflict with other rooms asking for heat. Confirm schedules, central controls and whether the outdoor unit is clear of stored items, snow or heavy debris from a safe position.
Book diagnosis if the unit never produces warm air, repeatedly enters defrost, shows an error, trips power, develops heavy persistent ice or heats much less than it did previously. Possible causes include sensors, airflow, valves, controls, outdoor-fan problems, refrigerant loss or compressor performance. A competent engineer should measure and diagnose rather than simply add refrigerant.
If the controls are unfamiliar, follow the air conditioner heat-mode settings guide. If the existing system no longer matches the winter job, compare repair with a properly designed replacement in the air conditioning heating guide. Do not assume a larger unit is the answer until room heat loss and low-temperature output have been checked.
Five safe checks before booking a repair
1. Read the controller. Confirm cooling mode rather than fan, dry or heating mode. Set the target several degrees below the room temperature and allow the system time to respond. Many systems deliberately delay compressor restart for a few minutes after being switched off.
2. Check the schedule and shared controls. A timer, weekly schedule, hotel-card switch, central controller, building-management system or open-window contact may override the local handset. Photograph any error code before resetting anything.
3. Inspect user-cleanable filters. Follow the manufacturer’s user instructions. A heavily blocked filter restricts airflow and can reduce cooling or contribute to icing. Do not remove fixed panels or reach into moving or electrical components.
4. Look for obvious airflow obstruction. Make sure curtains, furniture and stored boxes are not blocking the indoor intake or discharge. From a safe position, check that leaves, covers or stored materials have not enclosed the outdoor unit. Never reach through its grille.
5. Record what happens. Note mode, setpoint, room temperature, error code, which units are affected, when the fault began, recent building work and whether the outdoor unit starts. Clear information saves diagnostic time.
Do not repeatedly cycle the power, add refrigerant, bypass an electrical protection device or dismantle the unit. If those checks do not restore normal operation, use the detailed symptoms below and compare repair costs and qualified engineers.
Air conditioning running but not cooling
First decide whether airflow is weak or airflow is normal but not cold. Weak airflow points toward a dirty filter, blocked coil, fan problem, closed grille or duct restriction. Normal airflow at room temperature suggests controls, sensors, the outdoor unit, refrigerant circuit or compressor may not be delivering cooling.
One affected room on a multi-split or VRF system can indicate a local controller, sensor, valve, fan or drain-related problem. Every room affected at once points toward a shared outdoor unit, mode conflict, central control, power supply or network fault. Tell the engineer whether other zones work.
Hot weather alone does not prove the unit is undersized. A system that previously maintained temperature and suddenly cannot has probably changed condition. Dirty heat exchangers, lost refrigerant, outdoor recirculation or a failing component can reduce capacity. A system that has never cooled the room properly may instead be undersized, badly positioned or facing a heat load the original survey missed.
What an engineer may check
Expect a competent diagnosis to review controls and errors, temperatures, airflow, indoor and outdoor heat exchangers, fans, electrical readings, sensors and refrigeration performance. If refrigerant loss is suspected, the next step is leak diagnosis—not an unexplained top-up.
Air conditioning blowing warm air
Warm airflow commonly begins with the wrong operating mode. A sun symbol usually indicates heating; a fan symbol moves room air without mechanical cooling. Auto mode can also choose a different mode from the one expected. Select cooling, lower the setpoint and wait through any restart protection delay.
On shared systems, another controller may create a mode conflict. Some multi-splits and heat-pump VRF systems cannot heat one room while cooling another. The indoor fan may operate while the requested mode is unavailable. Check other controllers or the central system before assuming a refrigeration failure.
If settings are correct and the outdoor unit does not operate, there may be an electrical, control, sensor or outdoor-component fault. If it runs continuously but supply air never becomes cool, blocked heat exchange or a refrigeration problem is possible. Book a diagnosis rather than forcing repeated restarts.
Air conditioning leaking water indoors
Cooling naturally removes moisture from air. That condensate should collect in a tray and leave through a gravity drain or pump. Indoor dripping usually means the water path is blocked, displaced, damaged or unable to keep up. Dirty filters or coils can also contribute to freezing; when ice melts, the tray may overflow.
Switch the unit off if water approaches a socket, cable, light fitting or the unit’s electrical area. Protect finishes only if that can be done safely without opening the casing. Do not pour drain cleaner into the equipment or push tools into hidden pipework.
Tell the engineer whether the leak happens immediately or after extended cooling, whether a pump can be heard, and whether recent decorating or building work disturbed the unit or drain. Drain and pump problems are often repairable, but recurring leakage needs the route, fall, contamination and installation condition checked—not just the visible water wiped away.
Air conditioner smells musty, sour or burnt
Musty or stale smells commonly develop when dust and moisture remain on filters, coils, fan components or drains. A proper service may include cleaning appropriate components and correcting drainage. Simply spraying fragrance into the intake masks the symptom and can contaminate surfaces.
Drain or sewage-like odours can involve a dry trap, poor termination, stagnant condensate or another building drainage issue. The engineer may need to inspect how the air-conditioning drain connects, not only the indoor unit.
Burning, hot-plastic or sharp electrical smells require shutdown. Isolate the system safely if the controls and supply arrangement allow, and seek professional attention. Do not keep running it to see whether the smell clears.
Rattling, buzzing, clicking or grinding
Light plastic movement as temperatures change can be normal. New rattling may indicate a loose panel, filter or mount. Repeated condensate-pump noise can signal drainage problems. Buzzing can involve electrical components or vibration; grinding, impact or severe fan noise warrants prompt shutdown.
Record a short video from a safe distance and note the operating mode. Intermittent noises often disappear by the time an engineer arrives, and a recording can help identify whether the source is indoor, outdoor, pipework or drainage.
Air conditioning will not turn on or keeps tripping
Check whether the controller display works, batteries are fresh, the local isolator is in its normal position and other equipment has lost power. Do not remove electrical covers. A central controller, fire alarm, key-card switch or external interlock may hold the system off.
If a breaker or protective device trips repeatedly, leave it off and arrange an electrical and air-conditioning diagnosis. Repeatedly resetting protection can worsen a fault and creates risk. Tell the contractor what happens immediately before the trip: start command, compressor operation, heating changeover, rain, cleaning or other building work.
Error codes should be photographed exactly, including which lights flash and their sequence. Do not rely on an internet list alone: the same characters can mean different things across brands and controllers.
Ice on the indoor unit or refrigerant pipe
Ice can form when airflow is restricted, operating conditions are unsuitable, a sensor or fan is faulty, or the refrigeration circuit is not working correctly. Turn cooling off and allow the equipment to thaw naturally. Protect vulnerable finishes from meltwater, but do not chip ice away or apply direct heat.
A dirty user filter may be part of the cause, but visible ice deserves a broader check if it returns. Running a frozen system can reduce airflow further and send water outside the drain path when the ice melts.
Repair, service or replace?
A service is appropriate for routine cleaning, performance checks and maintenance. A repair is needed when a component, drain, control, electrical or refrigeration fault has been identified. Servicing should not be sold as a guaranteed cure for an undiagnosed breakdown.
If the system works but needs seasonal care, compare the scope and price of air conditioning and air-to-air heat pump servicing. A good service record should distinguish routine work from faults and recommended repairs.
Replacement becomes worth comparing when the system is old, unreliable, inefficient, uses difficult-to-support equipment, needs a major compressor or control repair, or no longer matches the room. Compare the cost and warranty of a repair with a properly surveyed replacement—not with an equipment-only online price. The repair cost guide includes a repair-versus-replace framework.
People working on applicable refrigerant systems must hold the correct qualifications described in current GOV.UK F-Gas guidance. Ask for a written diagnosis, proposed remedy, price and warranty before authorising substantial work.
Air conditioning fault FAQs
Why is my air conditioning running but not cooling?
Common causes include an incorrect mode or setpoint, dirty filters, blocked airflow, an outdoor-unit problem, sensor fault, refrigerant leak or compressor issue. Check user controls, filters and obvious airflow safely; persistent failure needs a qualified diagnosis.
Why is the indoor air conditioner leaking water?
A blocked condensate drain, dirty tray, failed pump, frozen coil or damaged pipe can cause indoor leakage. Switch the system off if water is near electrics or vulnerable finishes. Do not open the refrigeration circuit or dismantle electrical covers.
Why is my air conditioner blowing warm air?
It may be in fan or heating mode, waiting through a normal protection delay, affected by a dirty filter, or unable to run the outdoor refrigeration system. Confirm the controller settings and whether the outdoor unit is operating, then arrange diagnosis if cooling does not return.
Why is my air conditioner not heating?
Check that heating mode is selected, the setpoint is above room temperature and the system has had time to complete its normal start or defrost delay. Weak airflow, dirty filters, a multi-split mode conflict, outdoor-unit icing, sensors, valves, refrigerant or compressor faults can all affect heat. Persistent cold airflow or an error code needs a qualified diagnosis.
What causes an air conditioner to smell?
Dust and microbial growth on wet internal surfaces can create musty odours; a dry drain trap or drainage problem may cause other smells. Burning, sharp chemical or persistent electrical smells need prompt shutdown and professional attention.
Can I keep running a faulty air conditioner?
Stop if there is burning, smoke, sparking, repeated electrical tripping, water near electrics, severe vibration, damaged wiring or a suspected refrigerant leak. Minor performance loss is less urgent, but continued operation can worsen some faults. When unsure, switch off safely and seek advice.
Who can repair a refrigerant fault?
Work involving applicable fluorinated refrigerants requires the correct qualifications. Ask the company to identify the competent engineer and avoid anyone who proposes repeatedly adding refrigerant without finding and repairing the leak.
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