Most Common Air Conditioner Faults

We are still preparing products.

But you can look at other categories.

An air conditioner is one of those appliances we usually only notice once it stops working as it should - typically on the hottest day of July, when demand for cooling is at its peak and service companies have full schedules. Split air conditioners are, from a design point of view, relatively simple and reliable devices (compressor, two heat exchangers, fans, expansion valve, electronics), but that's exactly why most faults can already be recognised from their symptoms - you just need to know what to look for. In this article we go through the most common air conditioner faults in households and smaller premises in Slovakia the way we actually encounter them during installations and service call-outs: what causes them, what you can check yourself without tools, and when it's time to call a service technician instead of experimenting. At the end we also look at the question of when repairing an old unit is still worthwhile and when, from a cost perspective, replacing it with a new model is the better option.

The air conditioner is running but not cooling (or cooling weakly)

This is by far the most common reason people call for service. The indoor unit blows air, the display is lit, the remote control responds, yet nothing happens in the room, or the temperature difference is minimal. Across the hundreds of service call-outs we're familiar with from routine operation, the causes break down roughly as follows: thirty-five percent of cases are a dirty filter or clogged indoor unit heat exchanger, twenty-five percent are a refrigerant leak, fifteen percent are a blocked condensate drain (which also shows up as reduced output, not just dripping water), a further fifteen percent are an electronics or sensor fault, and the remaining ten percent are a mechanical compressor fault. Remember these proportions - we'll come back to them in the diagram below.

The first and simplest step is a visual check of the indoor unit's filter. Pull the filter out (for most wall-mounted units it's enough to lift the front panel) and hold it up to the light - if you can barely see light through it, it's clogged with dust, hair, or pollen and significantly restricts airflow through the evaporator. The result: the unit draws in less air, the evaporator gets locally over-cooled and may even frost up, and cooling output can drop by thirty to forty percent compared to a clean state. The fix is simple - wash the filter in lukewarm water (or with a mild detergent for heavier soiling), let it dry, and put it back. This is the one maintenance step users should do themselves, ideally once every two to four weeks during the season.

If the filter is clean and cooling is still weak, the next step is to check the settings - especially the mode (Cool, not Fan or Dry), the set temperature, and the fan speed. It seems trivial, but in practice it's surprisingly common for someone to accidentally switch the unit into dehumidify mode (Dry) or air circulation only (Fan Only) via the remote, then wonder why it's not cooling - in these modes the compressor either doesn't run at all or only intermittently.

If both the filter and the settings are fine and the unit still isn't cooling enough, the problem usually lies beyond what an ordinary user can handle - either there's a refrigerant shortage (typically a slow leak through a fitting or micro-cracks in the piping, which shows up as a gradual drop in output over weeks to months), or the outdoor unit's heat exchanger is so dirty it can no longer release heat to the surroundings. In both cases a certified refrigeration technician is needed - handling refrigerant legally requires professional qualification and special tools (a manifold gauge set, a vacuum pump); this is not something you can fix at home, nor should you attempt to.

The air conditioner is leaking or dripping water

The second most common complaint we encounter - water drips or even flows from the indoor unit, sometimes straight onto the floor or into the wall. The cause is almost always a blocked condensate drain. During cooling, the air conditioner condenses moisture from the air on the evaporator (the same principle as a misted-up car window), and this water has to drain out through a thin drainage hose, usually alongside the refrigerant piping to the outdoor unit or separately through the wall. Over time algae, dust, and biofilm build up in this hose, it becomes blocked, and instead of draining, the water overflows the internal tray and drips into the room.

A second possible cause is an incorrect slope of the drain hose - if the unit was installed without enough of a downward slope (or it shifted over time, for example during attic renovation work), water accumulates in the hose and eventually flows back. A third, less common cause is a frozen evaporator, which, on thawing (for example when the unit is switched off), produces more water at once than the drain can carry away - in this case, freezing is itself a symptom of another fault (low refrigerant, dirty filter) that needs to be addressed first.

What you can do yourself: on most units the outdoor end of the drainage hose is accessible (often near the outdoor unit or where the hose passes through the wall) - gently blowing air through it (for example with a hand pump, or carefully with a vacuum cleaner set to blow mode) can clear a minor blockage. For heavier blockages, or if the outlet isn't accessible, a technician needs to flush the hose with compressed air, or in extreme cases replace it.

The air conditioner is noisy or vibrating

A new air conditioner is practically inaudible under normal operation - the indoor unit at low speed reaches a noise level of around nineteen to twenty-two decibels, quieter than rustling leaves. If an unusual noise appears, it is always a signal that something is wrong, not "normal wear" to be ignored.

Creaking or knocking from the indoor unit

This is most often a loose plastic housing or front panel that vibrates against the unit's frame due to fan vibration. The fix is to tighten the mounting screws or clips, which you can usually do yourself after switching the unit off.

Buzzing or humming from the outdoor unit

The outdoor unit is inherently noisier (it contains the compressor); typical values are around forty-seven to fifty-two decibels, depending on the model's output. An increase in noise, or a new, unusual tone (metallic rattling, a high-pitched whine), most often indicates either loose mounting brackets (the unit isn't sufficiently isolated from the wall or base and is transmitting vibration into the building structure), worn fan bearings, or, in a worse case, a problem directly with the compressor. Loose brackets and rubber anti-vibration mounts can be tightened by a handy DIYer; a compressor repair is strictly a job for a service technician.

Knocking or bubbling in the piping

If you hear a sound resembling flowing or bubbling liquid directly in the copper piping between the units, it could be air in the system (for example after an improperly performed vacuum evacuation during installation) or the start of a refrigerant leak - refrigerant in gaseous state flowing through a partially leaking spot produces a characteristic hissing to bubbling sound. This always belongs in the hands of a service technician.

The air conditioner switches itself off or shows an error code

Modern air conditioners have built-in diagnostics and, when a fault occurs, display an error code on the indoor unit's screen or via a blinking indicator light (for example a combination of flashes, or a direct numeric/letter code such as E1, F3, and similar - the exact notation varies by manufacturer, so it's worth having the operating manual handy, or looking up the code for your specific model). Common causes of self-shutdown:

Compressor overheating - most often caused by a dirty outdoor unit (a clogged heat exchanger prevents heat dissipation) or operation at extremely high outdoor temperatures beyond the unit's specification. Solution: clean the outdoor unit's fins with a soft brush or a gentle stream of water (never a pressure washer directly on the fins - they would bend and lose effectiveness), and ensure free space around the unit (at least thirty centimetres on the sides, so it can freely draw in and expel air).

Communication error between the indoor and outdoor unit - often caused by a loose or damaged connecting cable, or moisture in the terminal box after a storm or after unqualified work on the wiring. Requires inspection by an electrician or service technician.

Low refrigerant pressure protection - the unit shuts the compressor off itself to prevent damage from a refrigerant shortage. This is essentially the same refrigerant leak we described above, just at a more advanced stage - instead of a gradual drop in output, the unit now repeatedly trips out.

One practical rule applies in all three cases: if the error code appears once and the unit then works fine afterwards, it may just be a one-off blip (for example a power outage, a storm). If the code recurs regularly, or the unit stops working entirely, you should contact service and report the exact code - this speeds up diagnosis and lets the technician arrive with the right spare part.

The outdoor unit is frosting up even outside winter mode

A light layer of frost on the outdoor unit during heating (heat pump/reverse) mode in winter is completely normal - the unit automatically defrosts it periodically by switching the cycle. The problem arises when the indoor unit (evaporator) frosts up during summer cooling, or when a thick layer of ice forms on the outdoor unit that doesn't melt even after several hours. The most common causes are the same as for weak cooling - insufficient airflow (clogged filter, blocked fins) or a low refrigerant level, which causes too low a temperature at the evaporator and freezing instead of evaporation. The fix starts the same way - switch the unit off, let the ice melt completely (never chip it off mechanically, you would damage the thin aluminium fins), check and clean the filter, and if the problem recurs, call a technician to check the refrigerant pressure.

Unpleasant smell from the air conditioner

A musty, "mouldy" smell when switching on the air conditioner is an extremely common occurrence, especially after a longer shutdown period (for example in spring after the winter break). The inside of the unit - the evaporator, the drip tray, the fan - is a permanently damp environment during operation, in which bacteria and mould multiply if cleaning is inadequate. On switching on, their smell is blown into the room. Prevention is simple and cheap: regular filter cleaning (see above), and once a year a thorough cleaning of the evaporator and drip tray - either DIY with a disinfectant spray made specifically for air conditioners (sprayed onto the fins with the unit switched off, according to the instructions), or professional steam cleaning by a service company, which also removes deposits deeper in the heat exchanger. If the smell persists even after cleaning, it may be a problem in the drainage hose itself (stagnant water) or, in a worse case, a dead rodent or insect stuck in piping routed through the wall - unusual, but it does happen occasionally in practice, especially with units that have long, rarely-used piping runs.

How often is service needed, and when is DIY maintenance enough

An ordinary household can get by with simple DIY maintenance (cleaning the filter every two to four weeks during the season, a visual check of the outdoor unit) plus one professional service inspection a year, ideally in spring before the cooling season starts. During this inspection the technician checks the refrigerant pressure, cleans both units' heat exchangers, checks the electrical connections and condensate drain - most of the faults described above can be caught in the early stages this way, before they cause more serious (and more expensive) damage, such as compressor seizure due to long-term low refrigerant.

When repair is still worth it, and when replacement is better

This is a question we hear very often from customers, especially with units older than seven to ten years. The general rule we also use for our recommendations: if the estimated repair cost is higher than half the price of a new comparable unit, or if it's a compressor fault on an older unit (the compressor is the most expensive part; replacing it typically costs several hundred euros including labour and new refrigerant), a repair generally isn't worth it. On the other hand, minor repairs - cleaning, tightening brackets, replacing the drain hose, topping up refrigerant for a small leak - are cheap (typically tens to low hundreds of euros) and extend the unit's life by years; in these cases a repair is worth it almost always.

The unit's age and refrigerant type also play a role in the decision - older units running on R22 refrigerant (now banned for new sales in the EU) are practically never worth topping up in the case of a bigger leak, since the refrigerant itself is hard to obtain and expensive. New units using R32 refrigerant (for example the Vaillant climaVAIR series we offer) don't have this problem and also have significantly higher energy efficiency, so replacing an old unit brings not just a fix but also savings on running costs.

Recommended units, if repair is no longer worth it

If your old air conditioner has reached the point where repair exceeds a reasonable cost threshold, or you're simply looking for a reliable replacement without the risk of repeated faults, we recommend looking at the Vaillant climaVAIR Pro series - wall-mounted split units with modern R32 refrigerant, quiet operation, and good availability of spare parts and service in Slovakia.

VAIB1-025WN outdoor and indoor unit with remote control - output 2.7 kW, a suitable replacement for an older unit in an ordinary room or smaller living room, quiet operation and low consumption thanks to a modern compressor. Price from €527.00.

VAIB1-035WN outdoor and indoor unit with remote control - a mid-power step, a good choice if you're replacing a faulty unit in a larger room or a space with larger glazed areas. Price from €561.00.

VAIB1-050WN outdoor and indoor unit with remote control - the highest output in the series, suitable for larger open spaces or rooms with a higher heat load, where an older, faulty unit had long since stopped keeping up. Price from €833.00.

Frequently asked questions

Why does my air conditioner only blow cold air without actually cooling the room?

If the air coming from the grille feels "cold" to the touch but the room isn't cooling, this is most often too low an output for the size and heat load of the room (for example large south-facing glazing), or a partial refrigerant leak, where the unit still cools, but at significantly lower output than it should. We recommend checking the filter as the first step, and if it's clean, having the refrigerant pressure checked by a service technician.

Is it normal for water to drip from the outdoor unit in summer?

Yes, a slight dripping of condensate from the outdoor unit during cooling is common and harmless - it's moisture condensation, similar to the indoor unit. The only real problem is significant leaking inside the building, or ice forming on the outdoor unit outside of winter heating mode.

How often does refrigerant in an air conditioner need to be topped up?

A properly functioning, leak-free air conditioner neither consumes nor loses refrigerant - it's a closed circuit. If refrigerant needs to be topped up repeatedly (for example every year), it means there is a leak somewhere in the system that needs to be found and sealed, not a problem to keep solving by repeatedly topping up indefinitely.

Can I leave a faulty air conditioner running until the technician arrives?

With weaker cooling or a minor condensate drip, yes, the unit won't be significantly harmed by that. With repeated shutdowns via an error code, freezing, or unusual noises from the compressor, we recommend switching the unit off instead and waiting for the technician - running it in a faulty state can cause further, more expensive damage (for example compressor seizure due to a refrigerant shortage).

Why does the air conditioner start smelling after a few years even though I clean it regularly on the outside?

Ordinary wiping of the outer casing won't remove the smell, since the source is inside the unit (evaporator, drip tray), which you can't reach without disassembling the panels. Once a year we recommend either a disinfectant spray made specifically for air conditioner evaporators, or professional steam cleaning by a service company.

Is it worth signing a service contract for regular inspections?

With one or two units in a household, most people manage fine by booking a one-off service inspection once a year without a contract. A service contract makes more sense with multiple units (for example office premises) or if you prefer an automatic reminder and priority scheduling during the season.

Do you have a question about this topic?

Dealing with a specific fault in your air conditioner, or not sure whether repair is still worth it? Write to us - we're happy to help.