The air conditioner is not cooling: 12 causes, checks and solutions
Learn to distinguish between a system that blows air but not cold air, a unit that cools poorly, a unit that stops cooling after a while, a frozen pipe or an outdoor unit that does not start.
“Not cooling” can describe very different problems
An air conditioner can remain on and move air even when the compressor or outdoor unit is not producing cooling. It can also work correctly at first and then lose performance due to ice, insufficient airflow or an operating protection.
For this reason, the first step is not to assume that it “needs gas”, but to observe exactly what is happening: whether the airflow is normal, whether the air comes out cold, whether the outdoor unit is running, whether ice appears, whether there is an error code and whether the room has ever reached the requested temperature.
The checks in this guide are intended mainly for split systems and air-to-air heat pumps. In ducted systems, multisplit systems or commercial installations, dampers, duct networks, centralised controls or several indoor units may also be involved.
What to check if the air conditioner is not cooling
If the air conditioner is running but not cooling, first check that it is in Cool mode, that the setpoint is lower than the room temperature, that the filters and grilles are unobstructed, and that the outdoor unit starts operating after the start-up delay. If there is ice, the outdoor unit does not start, an error code appears or lukewarm air continues to come out after the basic checks, there may be an airflow, refrigerant, fan, electronics or compressor problem that requires professional diagnosis.
| Symptom | Check first | Possible causes |
|---|---|---|
| Air comes out but is not cold | Mode, setpoint, start-up delay and outdoor unit. | Configuration, outdoor unit stopped, refrigeration circuit, electronics or compressor. |
| Cools poorly | Filters, airflow, doors and windows, outdoor heat exchanger. | Dirt, high heat load, refrigerant, distribution or sizing. |
| Cools and then stops | Airflow, ice, error code and outdoor unit operation. | Freezing, protection, fan, sensor, electronics or refrigeration control. |
| The outdoor unit does not start | Protection delay, actual cooling demand and error codes. | Power supply, communication, electronics, fan or compressor. |
| Ice appears | Filters, airflow and condition of the indoor coil. | Dirt, fan, insufficient refrigerant or refrigeration-circuit control. |
First identify exactly how the air conditioner is behaving
Select the symptom that most closely matches what you observe. The summary below will tell you where to start checking.
Summary based on the symptom
You can also change the symptom using the selector.
Start with configuration, airflow and the outdoor unit
Check that the unit is in cooling mode, that the setpoint is below room temperature, that the filters are clean and that the outdoor unit starts operating.
A safe ten-minute check before requesting service
This sequence rules out incorrect settings, visible obstructions and basic problems without opening casings or handling electrical components or the refrigeration circuit.
Carry out the checks in this order
Changing several things at the same time makes it difficult to know what the cause was. After adjusting the unit, allow it time to stabilise.
The 12 most common causes of an air conditioner not cooling
The first checks are accessible to the user. Causes related to electricity, refrigerant, sensors, internal fans, refrigeration control or the compressor require professional diagnosis and tools.
Incorrect mode, setpoint or timer
The unit may be in fan-only, dehumidification, heating or automatic mode. It may also have a setpoint above the room temperature, an active schedule or a limitation set in the controller.
Dirty or clogged indoor filters
Accumulated dust reduces the air drawn in, lowers the supply airflow and may increase the time and energy required to condition the space.
Blocked air intake or outlet
Curtains, furniture, suspended ceilings, closed grilles or poor recirculation may prevent the unit from drawing in and distributing enough air.
Dirty indoor evaporator or blower wheel
Even if the filters are clean, the evaporator coil or blower wheel can accumulate adhered dirt and lose heat-exchange or air-moving capacity.
Dirty outdoor heat exchanger or insufficient ventilation
The outdoor heat exchanger, which acts as the condenser during cooling, needs sufficient airflow to reject heat outdoors. Leaves, dust, vegetation, objects or an excessively enclosed location can hinder this process.
Frozen evaporator or pipe
Insufficient airflow or a refrigeration-circuit anomaly can cause the evaporator temperature to drop too far and turn condensed moisture into ice.
Refrigerant leak or incorrect charge
The refrigerant circulates inside a closed circuit. An insufficient amount may result from a leak, an incorrect initial charge or a previous intervention carried out incorrectly.
The outdoor unit is not receiving a command or power supply
There may be a power-supply, communication, protection, wiring, control-electronics or configuration problem that allows the indoor fan to run while preventing the outdoor assembly from operating.
Faulty indoor or outdoor fan
A motor, electronic module or mechanical problem may prevent a fan from rotating at the required speed even though other parts of the unit remain active.
Compressor, electronics or control fault
A thermal protection, the compressor, inverter module, electronic board, sensor, communication between units or a refrigeration-circuit control element can interrupt or limit cooling production.
In installations with a condensate pump or level control, a condensate alarm can also inhibit cooling operation.
Very high heat load or outdoor temperature
Direct sunlight, large glazed areas, many people, electrical equipment, open doors, air infiltration or a very hot building envelope can temporarily exceed the available capacity.
The operating range of the model must also be taken into account. At very high outdoor temperatures, available capacity may decrease and some units may limit operation or activate protections. Always consult the limits specified by the manufacturer.
Undersized unit or poor installation
Insufficient capacity, poor distribution, pipe lengths or height differences outside the specified limits, a refrigerant charge not adjusted to the pipe run when required, poor insulation, leaking ducts or an unsuitable location can limit performance from the first day.
What to check based on the unit's exact behaviour
These paths go deeper into the four most common search patterns. If ice, a fault code or any risk signal appears, you do not need to complete the entire sequence before stopping the unit.
It blows air, but not cold air
The indoor fan can operate independently of effective cooling production.
- Confirm cooling mode and cancel timers.
- Set a setpoint below the room temperature.
- Allow for the protection delay after start-up or a mode change.
- Check whether the outdoor unit starts operating.
- Look for an error code, ice or repeated shutdown.
- If lukewarm air continues to come out, request a technical diagnosis.
It cools, but not enough
When there is some cooling, the problem may be related to airflow, heat exchange, distribution, heat load or capacity.
- Check the filters and clean them according to the manufacturer's instructions.
- Clear the air intake, supply outlet and return grilles.
- Check that doors and windows are closed.
- Reduce direct sunlight and internal heat sources.
- Observe the outdoor unit and the outdoor temperature.
- If it never reaches the setpoint, review maintenance and sizing.
It cools and then stops cooling
Observe whether only the cooling disappears, whether the airflow also decreases, or whether the entire unit stops.
- Check whether airflow decreases progressively.
- Look for ice on the coil, under the casing or on the pipes.
- Observe whether the outdoor unit stops while the indoor unit continues blowing air.
- Note how long it takes for the problem to appear.
- Check filters and obstructions before testing again.
- If it repeats, there may be a protection, sensor, fan, electronics or refrigeration anomaly.
The outdoor unit does not start
The outdoor unit may be delayed for a few minutes, but it should not remain stopped indefinitely if there is an actual demand for cooling.
- Wait for the protection period after switching on or restarting the unit.
- Verify that the indoor unit is in cooling mode and requesting a lower temperature.
- Check whether there is an error or a flashing light.
- Visually inspect the accessible electrical protection devices.
- Do not keep trying if the circuit breaker trips again.
- Request inspection of the power supply, communication, electronics, fan and compressor.
What it means when the pipe or indoor unit freezes
Ice does not mean that the unit is “cooling more”. It indicates that the evaporator is operating under unsuitable conditions and may eventually partially or completely block the airflow.
Do not continue using cooling mode
Keeping the compressor running while the evaporator is frozen does not solve the cause. In addition, when the ice melts, a considerable amount of water may appear.
Do not use knives, tools, very hot hair dryers, steam or boiling water. You could deform the fins, puncture a pipe or wet electrical components.
What the user can check and what should be handled by a professional
A visual check and basic cleaning are not equivalent to diagnosing the electronics or refrigeration circuit.
Checks you can perform without dismantling anything
Always consult the specific manual, as access to filters and permitted operations may vary by model.
- Check the mode, setpoint, clock and timers.
- Replace the remote-control batteries when necessary.
- Clean removable filters according to the instructions.
- Clear indoor air intakes and outlets.
- Remove accessible external debris around the outdoor unit.
- Note error codes, flashing lights, noises and operating times.
- Check doors, windows, solar exposure and heat sources.
Operations that are not household maintenance
These operations involve electrical voltage, moving parts, pressure, refrigerant or specific measuring instruments.
- Measure or correct the refrigerant charge.
- Find, repair and verify the leak tightness of a refrigerant leak.
- Open the outdoor unit or access its components.
- Measure capacitors, windings, boards, currents or insulation.
- Replace fans, sensors, valves, inverter components or the compressor.
- Internally clean the blower wheel or evaporator when dismantling is required.
- Modify refrigerant pipes, drains or electrical connections.
Request assistance and stop using the unit if there is a burning smell, smoke, sparks, electrical protection devices tripping, severe mechanical noises, water on electrical components, recurring ice or repeated attempts by the outdoor unit to start. If there is an electrical risk, disconnect the power only if you can do so safely.
How to reduce the likelihood of the unit losing capacity again
Maintenance does not prevent every fault, but it helps preserve airflow and heat exchange and makes it possible to detect anomalies before the unit stops cooling completely.
Check the filters
The frequency depends on use, dust, the environment and the manufacturer's instructions. Do not wait until they are completely clogged.
Keep the grilles clear
Avoid furniture, curtains, boxes or decorative items in front of the intake, supply outlet or returns of a ducted system.
Observe the outdoor unit
Remove accessible leaves and debris and check that hot air can leave without immediately recirculating back into the same unit.
Schedule inspections
An inspection can detect internal dirt, problematic drains, deteriorated connections, abnormal values or signs of a possible leak.
Lowering the setpoint significantly does not increase the available capacity
On many units, selecting an extremely low temperature does not make the system produce more capacity than it was designed for. It mainly keeps the demand active for longer until the setpoint is reached or a limitation prevents it from being achieved.
Frequently asked questions
Why does air come out, but it is not cold?
The indoor fan can run even when there is no effective cooling production. Check the mode, setpoint, filters and outdoor unit. If the outdoor unit does not start or the air remains lukewarm, there may be a control, refrigerant, fan, electronics or compressor problem.
Why is the air conditioner cooling poorly?
Common causes include dirty filters or heat exchangers, obstructions, poor distribution, excessive heat in the space, an outdoor unit with poor ventilation, a refrigeration anomaly or insufficient capacity.
Why does it cool at first and then stop?
Ice may appear on the evaporator, airflow may progressively decrease, the outdoor unit may stop for protection, or there may be a fan, sensor, electronics, compressor or refrigeration-control fault.
Is it normal for the outdoor unit to take time to start?
Yes, on many units. After a shutdown, restart or mode change, there may be a delay of approximately three minutes to protect the system. The exact time depends on the manufacturer and model.
What should I do if the pipe is frozen?
Stop cooling mode and allow the ice to melt completely. Check the filters and airflow obstructions. Do not break the ice or apply intense heat. If it appears again, request an inspection.
Is it normal to have to recharge the refrigerant every summer?
No. Refrigerant operates inside a closed circuit. A recurring loss of charge indicates that leak tightness must be checked, the cause determined and any possible leak repaired where appropriate.
Can I wash the outdoor unit with a hose?
Applying water indiscriminately is not recommended. Do not use a pressure washer or pressurised water: the fins are delicate and there are electrical components. You can remove accessible external debris with the unit stopped; deep cleaning should be carried out according to the manufacturer's instructions or by professional personnel.
When might the unit be too small?
When it runs for many hours without reaching the setpoint, especially with high solar load or occupancy, even though it is correctly configured and in good condition. The heat load should be calculated and the capacity, distribution and installation reviewed.
Should I restart the unit several times?
Repeated restarts are not advisable. You can perform a single check following the manual and allowing for the protection delay. If the error reappears, note the code and request assistance.
Can a dirty filter cause the unit to freeze?
Yes. A heavily clogged filter reduces airflow across the evaporator. Under certain conditions, the coil temperature can drop excessively and the condensed moisture can turn into ice. However, if ice reappears with clean filters, other causes should be considered.
