Air conditioner faults — by symptom
Describe what the unit is doing and the cause is usually clear from that alone. Below are the faults I get called out to most often, what causes them, and what you can check yourself before picking up the phone.
Symptom
AC is not cooling
The unit runs, the fan blows, but the air is lukewarm or barely cool. This is the call I get most often, and in the vast majority of cases the compressor is not the problem — it is something far simpler.
First I measure the air temperature at the inlet and outlet of the indoor unit. A difference under eight degrees means the system is not working as it should. Then I move on to pressures and look for the cause — I do not top up refrigerant until I know where it is going.
Most common causes
- Clogged filter or evaporator
- A layer of dust physically blocks the airflow. The most common cause and the cheapest to fix.
- Low refrigerant from a leak
- An AC does not consume refrigerant. If it is low, it is escaping somewhere — usually at a joint or on the copper line.
- Dirty condenser on the outdoor unit
- The outdoor unit cannot reject heat, so the whole cycle chokes.
- Wrong operating mode
- The unit is set to fan instead of cooling. Check before you call — it would not be the first time.
Symptom
AC is leaking water indoors
Water drips from the indoor unit, runs down the wall or collects underneath. It sounds serious, but it is almost always a blocked condensate drain — a quick job, not an expensive fault.
While cooling, an AC pulls moisture out of the air and that water has to go somewhere. It collects in the tray under the evaporator and runs out through a hose. When that hose clogs with sludge or algae, the tray overflows into the room.
Most common causes
- Blocked condensate drain
- By far the most common cause. The hose gets flushed and cleared under pressure.
- Indoor unit not level
- If the unit was not set with a slight fall towards the drain, water runs back inside. An installation error.
- Frozen evaporator thawing
- Ice builds up from low refrigerant or a clogged filter, then melts all at once when the unit stops.
- Disconnected or cracked hose
- Behind the cover or inside the wall, where you cannot see it. The water goes into the wall instead of the drain.
Symptom
AC is not heating in winter
The unit is set to heating but blows cold air, or shuts down after a few minutes. On inverter units used for heating as well, this is the typical winter call.
Before anything else I check whether it is a normal defrost cycle — an AC has to melt the ice off the outdoor unit from time to time, and stops heating for a couple of minutes while it does. That is not a fault. If the pause lasts longer or repeats constantly, diagnostics follow.
Most common causes
- Reversing valve
- The part that flips the cycle between cooling and heating. When it sticks, the AC stays in cooling mode.
- Low refrigerant
- A shortage shows up more strongly on heating than on cooling, which is why it is often first noticed in winter.
- Iced-up outdoor unit
- If the ice does not clear properly, the coil cannot draw heat from the outside air.
- Outside temperature too low
- Older non-inverter models practically stop heating below around −5 °C. That is the limit of the unit, not a fault.
Symptom
AC is noisy, rattling or vibrating
The unit runs and cools normally, but makes a noise it did not make before. It is worth acting early — vibration left alone destroys brackets and bearings, and that is a more expensive repair.
The sound often tells you where the problem is. Rattling is usually mechanical and loose, a whine points to a bearing or the fan, and a dull drone to the compressor or the brackets.
Most common causes
- Brackets without anti-vibration pads
- The outdoor unit passes vibration straight into the wall and the whole facade rings.
- Loose screws and cover
- After years of running, parts work loose and start rattling on every start-up.
- Fan touching the housing
- A bent blade or a shifted bearing seat. The scraping follows the rhythm of the rotation.
- Foreign object in the outdoor unit
- A leaf, a twig or a nest. More often than you would think.
Symptom
AC smells when it starts
A damp, musty, “basement” smell for the first few minutes after switching on. This is not a cosmetic problem — what you are smelling is mould and bacteria that the unit is spreading around the room you sleep in.
The evaporator is permanently damp, dark and dusty. That is an ideal environment for mould, especially in the tray and on the blower wheel where sludge builds up over years. The filter can be rinsed at home, but the evaporator and the tray can only be reached by dismantling.
Most common causes
- Mould on the evaporator
- Visible as a dark deposit between the fins. Rinsing the filter does not fix it.
- Sludge in the condensate tray
- Standing water mixed with dust. The main source of the smell.
- Dirty blower wheel
- A deposit on the blades that gets blown around the room on every start.
- Dry trap on the drain
- On ducted and ceiling units the smell can come from the drainage system itself.
Symptom
Error on the display or the AC shuts itself off
An error code is lit on the indoor unit, the lights blink in a sequence, or the unit starts and then switches itself off after a few minutes. Codes differ between manufacturers, so tell me the make and exactly what it says.
When an AC shuts itself down, that is usually a protection doing its job — the electronics measured something out of range and cut the compressor before it could be damaged. I find the cause by measuring, not by swapping parts blindly.
Most common causes
- Overheating from a dirty outdoor unit
- The condenser cannot shed heat, pressure climbs, the protection cuts the compressor.
- Faulty temperature sensor
- The electronics get a wrong reading and act on it.
- Power supply problem
- Voltage drop, a bad connection at the terminals, or a break in the cable to the outdoor unit.
- Indoor unit electronics
- Capacitors and relays age. Sometimes the board can be repaired, sometimes it has to be replaced.
Symptom
AC will not start at all
No response to the remote, the display is dead, nothing can be heard. Before you call, there are a few things that take two minutes to check and that account for a good share of these calls.
If the batteries, the breaker and the switch are all fine and the unit is still dead, then it is on to checking the power supply and the electronics on site.
Most common causes
- Batteries in the remote
- It sounds trivial, but it is a frequent case. Check whether the remote display lights up at all.
- Tripped breaker
- Look at the board. If the breaker trips again after you reset it, do not reset it again — call.
- Broken switch or connection
- On installations where the AC runs through a dedicated switch or a socket behind the unit.
- Indoor unit electronics
- If no voltage reaches the board, or the board is faulty, the unit does not wake up.
Common questions
Questions about faults
Why is my AC not cooling even though it runs?
In most cases the cause is a clogged filter or evaporator, a dirty condenser on the outdoor unit, or low refrigerant from a leak. First I measure the temperature difference between inlet and outlet — under eight degrees means the system is not working properly. I do not top up refrigerant until I find where it is escaping.
Why is my AC leaking water into the room?
Almost always the condensate drain is blocked, so the tray under the evaporator overflows into the room. Less often it is a wrong tilt of the indoor unit from the installation, or a frozen evaporator thawing all at once. Switch the unit off so water does not reach the electronics and put a container underneath until the call-out.
Why is my AC not heating in winter?
First I check whether it is a normal defrost cycle — an AC has to melt ice off the outdoor unit periodically and stops heating for a couple of minutes while it does. If that is not it, the usual causes are a stuck reversing valve, low refrigerant, or an iced-up outdoor unit. Older non-inverter models practically do not heat below −5 °C.
Why does my AC smell bad?
The smell comes from mould and bacteria on the evaporator and sludge in the condensate tray. Rinsing the filter does not fix it because the source is deeper inside the unit. It needs a service clean with dismantling and washing of the evaporator, blower and tray, followed by disinfection.
What does the error code on my AC mean?
Codes differ between manufacturers, so tell me the make, the model and exactly what the display says or how the lights blink. When an AC shuts itself off, that is usually a protection that measured something out of range and cut the compressor before damage.
My AC will not start — what can I check myself?
The batteries in the remote, the breaker on the board, a dedicated switch if there is one, and whether the socket works with another appliance. If the breaker trips again after resetting, that is a sign of a short circuit — do not force it on, it can be dangerous.
AC died and the weather is hot?
Describe the symptom by phone or message. For most faults I know from the description what to bring, so the job gets finished in one visit.