How to recognise common air conditioning faults, what you can safely check yourself, what a qualified engineer does on a repair visit, and when replacing an older system makes more sense.
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When an air conditioner stops cooling properly, the reason is often simpler than it looks. You'd be surprised how many call-outs turn out to be a blocked filter, a tripped breaker or a remote set to the wrong mode. Other faults need a qualified engineer with the right test equipment. Refrigerant leaks, failing fan motors and electronic board problems all fall into that group.
This guide is about diagnosis. We go through what the common symptoms usually mean and which checks you can safely do yourself. Then we cover what an engineer does on a visit and how to decide if an older system is worth repairing. It applies to the split, multi-split, ducted and cassette systems found in UK homes and workplaces.
Most faults show up as one of a handful of symptoms. The table below matches each symptom to its usual causes. Treat it as a pointer and not a diagnosis, because several different faults can look the same from the outside.
| Symptom | Likely causes | Owner can check? |
|---|---|---|
| Unit will not switch on | Tripped breaker or isolator, flat remote batteries, control board fault | Partly: breaker, isolator and batteries |
| Runs but blows warm or room-temperature air | Wrong mode selected, dirty filters, low refrigerant from a leak, outdoor fan or compressor fault | Partly: mode and filters |
| Water dripping from the indoor unit | Blocked condensate drain, failed condensate pump, unit not level, ice melting off the coil | No, beyond switching off and protecting the floor |
| Ice on pipework or the outdoor unit in cooling mode | Restricted airflow or low refrigerant charge | Partly: filters and airflow |
| Rattling, buzzing or grinding | Loose panel, debris in the outdoor fan, worn fan bearings, failing compressor | Only a visual check for debris with the power off |
| Musty or damp smell | Biological growth on the indoor coil or in the drain tray | Filters only; coil cleaning needs an engineer |
| Flashing light or error code | The unit's self-diagnosis has detected a specific fault | Note the code and tell the engineer |
Modern splits are heat pumps, so a few things that look like faults only happen in heating mode, and they're perfectly normal. In cold, damp weather the outdoor unit runs a defrost cycle every so often to clear ice off its coil. While it does, the indoor fan may stop for a few minutes and the outdoor unit may give off a cloud of vapour that looks like smoke. Neither is anything to worry about. If the outdoor unit stays encased in ice, or the indoor unit never returns to heating after defrost, that does need an engineer.
A few simple checks can save you a wasted visit. You won't need to open the unit or touch anything electrical apart from a switch.
If the unit is leaking water, smells of burning, or trips the electrics, switch it off at the isolator and do not keep using it until it has been inspected.
Water leaking from an indoor unit is almost always a condensate problem. It's very rarely anything to do with the refrigerant. As the unit cools, moisture in the room air condenses on the cold coil, collects in a tray and drains away through a pipe, by gravity or with a small pump. If anything interrupts that path, you get a leak.
Usually it's a drain clogged with dust and biological growth, or a condensate pump that has died. A drain pipe that has lost its fall will do it too, and so will an indoor unit that wasn't fitted level. Low refrigerant can also make the coil freeze, and when the ice melts there's more water than the tray can handle. An engineer will clear and flush the drain, test the pump and check the system pressures to rule out the refrigerant side.
A sealed air conditioning system doesn't use up refrigerant. If the charge is low, it's leaking somewhere. Adding more gas without finding the leak only buys a little time while the refrigerant escapes into the atmosphere again, and it isn't good practice under the F-Gas rules.
A proper refrigerant repair follows a set order. The engineer tracks down the leak with detection equipment and recovers whatever refrigerant is left. Next comes the repair to the joint or component, then a nitrogen pressure test. After that the circuit is evacuated under vacuum to get the moisture out and recharged with the correct amount of refrigerant. Only F-Gas qualified engineers working for a certified company are allowed to do this.
Older systems still running on R22 refrigerant are a special case. R22 can no longer be used to service systems in the UK, so a leak on an R22 unit usually means replacing it, and system replacement is one of the air conditioning services we offer.
When an engineer from UK Aircon arrives, the first thing they do is ask how the fault started. They read any error code and check the basics before getting the tools out. After that they test the electrical supplies and components, measure pressures and temperatures, and look over the fans, coils and drains. The point is to find what's actually causing the problem so it doesn't come straight back.
Once the fault is found, you should be told what has failed and why, and what the repair involves. If a part has to be ordered, the engineer can usually make the system safe until it arrives. And if the fault suggests the system is close to the end of its life, you should hear that honestly.
After the repair, the system should be run in cooling and heating, readings rechecked and the room temperature confirmed. That's how knock-on problems get caught, like a drain that blocks again or a second weak component. If you need someone to come out, our air conditioning near me page lists the areas we cover.
On a system that's only a few years old, most faults are worth repairing. The older the equipment, the harder the call. Think about replacement when:
A modern inverter split on R32 is generally more efficient than a unit from ten or more years ago, and it heats well as well as cooling. If replacement is the better option, see our guide to air conditioning installation for how a new system is planned.
Before you book anyone, check they hold F-Gas company certification through a body such as REFCOM and that their engineers are qualified to handle refrigerant. They should also explain the fault and the cost clearly before they start. Be wary of anyone who offers to "regas" a system without looking for the leak, or who can't tell you which refrigerant your unit uses.
UK Aircon's technicians are background checked and trained every month. They service air conditioners from any manufacturer, so it doesn't matter if we didn't install yours.
Repair costs depend on the fault, so nobody can give you a reliable figure until it's been diagnosed. The main factors are the time needed to find the fault, the price and availability of parts, whether refrigerant work is involved, how easy the units are to reach (high-level outdoor units may need access equipment), and the size and type of system. Cassette and VRF systems in commercial premises generally take longer to work on than a single domestic split.
Of course, the cheapest repair is one you never need. Regular servicing picks up blocked drains, worn fan bearings and weak components before they fail. Our guide to air conditioning maintenance explains what a service covers and how often it is needed.
Common causes are the wrong mode on the remote, blocked filters, an obstructed outdoor unit, low refrigerant from a leak, or a fan or compressor fault. Check the mode, filters and outdoor unit first. If cooling does not improve, an engineer needs to test the system.
It is best to switch it off. Water leaking from an indoor unit can damage walls, floors and electrics below, and the cause, usually a blocked drain or failed pump, will not fix itself. Protect the area and arrange an inspection.
No. Handling refrigerant legally requires an F-Gas qualified engineer working for a certified company. Low refrigerant also means there is a leak, which must be found and repaired before the system is recharged.
UK Aircon's technicians service and repair air conditioners from any manufacturer, including systems installed by other companies, although parts availability for older or discontinued models can affect what is possible.
Electrical and mechanical faults can sometimes be fixed, but R22 can no longer be used to service systems in the UK. Any fault that requires refrigerant work on an R22 system usually means replacing it with a modern unit.
For systems installed by UK Aircon, the 5-year installation warranty covers parts and labour and includes access to 24-hour emergency call-out. For other systems, check the installer's and manufacturer's warranty terms.
If your system is not cooling, heating, draining or running as it should, our maintenance and repairs team can diagnose the fault and explain the options clearly. Get in touch for a free quote and we will reply within 24 hours.
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