Air conditioner won’t start: Causes and what to do

If your air conditioner won’t start, the problem may be something simple such as the remote control, the electrical supply, or an incorrect setting. In other cases, there may be a fault in the outdoor unit, the electronics, or the compressor.

Before calling a technician, there are a few basic checks you can carry out without opening the unit or handling electrical components.

What to check if your air conditioner won’t start

Check the power supply

If the air conditioner does not turn on and no lights appear on the indoor unit, start by checking the electrical supply.

Check:

• The power outlet, if accessible.
• The air conditioning isolator switch.
• The electrical panel.
• The circuit breaker for the air conditioning system.

The circuit breaker, sometimes simply referred to as the breaker or automatic switch, may trip because of an overload or an electrical fault.

You can reset it once. If it trips again when the air conditioner is switched on, do not keep resetting it. The system should be checked by a qualified technician.

Check the remote control

If the system does not respond, replace the batteries in the remote control before assuming there is a more serious fault.

Also make sure there are no objects blocking the receiver on the indoor unit.

You can sometimes test the infrared signal using your smartphone camera. Point the remote towards the camera and press a button. On many phones, you will see a small light on the screen if the remote is transmitting a signal.

Check the operating mode and temperature

Sometimes the air conditioner is switched on but the compressor does not start because the current settings do not require cooling or heating.

If you want to cool the room:

• Select cooling mode.
• Set a temperature lower than the current room temperature.
• Make sure the unit is not set to fan-only mode.
• Check that no timer is active.

For example, if the room temperature is 25°C and the system is set to 27°C, the indoor unit may remain on without starting the cooling cycle.

Wait a few minutes

If you have just switched the system off and turned it back on immediately, the outdoor unit may take a few minutes to start.

Many air conditioning systems include a compressor protection delay to prevent repeated starts within a short period.

During this time, the indoor unit may appear to be operating while the outdoor unit remains stopped.

Wait a few minutes before assuming there is a fault.

Air conditioner does not turn on at all

If the unit is completely off and does not respond to the remote control, the most common causes include:

• No electrical supply.
• Tripped circuit breaker.
• Isolator switch turned off.
• Flat remote-control batteries.
• Blown internal fuse.
• Faulty electronic control board.

If there is power to the property and the remote is working correctly, but the air conditioner remains completely unresponsive, a technical inspection may be necessary.

Air conditioner turns on but does not start

Another common situation is when the air conditioner turns on but does not start cooling or heating.

The indoor unit may light up, respond to the remote, or even move air, but the system does not begin conditioning the room.

First check:

• The selected operating mode.
• The set temperature.
• Any active timers.
• Whether several minutes have passed since switching it on.
• Whether an error code appears.

If everything is configured correctly, the problem may be located in the outdoor unit.

Indoor unit works but outdoor unit does not start

If the indoor unit is running but the outdoor air conditioning unit does not start, the possible causes include:

• Electrical supply problems.
• Communication faults between the indoor and outdoor units.
• Compressor protection mode.
• Faulty electronic control board.
• Outdoor fan fault.
• Compressor problems.

It is also useful to listen for unusual noises.

A humming sound, repeated attempts to start, or any sound that is different from normal operation can help a technician identify the source of the problem.

Do not open or attempt to repair the outdoor unit yourself.

Why won’t the air conditioner compressor start?

The compressor is one of the main components of the air conditioning system. If it does not operate, the system cannot complete the refrigeration cycle correctly.

If the air conditioner compressor won’t start, several components may be responsible.

Electrical problem

Incorrect voltage, damaged wiring, or an activated electrical protection device can prevent the compressor from operating.

Electronic control board

The control board manages the operation of the system. If it fails, the signal required to start the compressor may not be sent correctly.

Temperature sensors

Temperature sensors provide information that the system uses to control operation.

A faulty sensor may send incorrect readings and cause the air conditioner to operate abnormally or activate a protection mode.

Capacitor

In some air conditioning systems, a faulty capacitor can prevent the compressor or outdoor fan from starting properly.

One possible symptom is a humming sound coming from the outdoor unit without the system fully starting.

Compressor fault

The compressor itself may also develop an electrical or mechanical fault.

Correctly diagnosing a compressor problem requires specific measurements and testing equipment, so this type of fault should be assessed by a professional technician.

Could low refrigerant cause the problem?

A refrigerant leak can affect the performance of an air conditioning system, but an air conditioner that will not start does not automatically need more refrigerant.

Electrical faults, electronic components, sensors, or compressor problems should also be considered.

If the system does have a refrigerant leak, simply adding more refrigerant without first locating and repairing the leak will only provide a temporary solution.

What can you check yourself?

There are several simple checks that can be carried out safely:

• Replace the remote-control batteries.
• Check the electrical panel.
• Check the circuit breaker.
• Confirm the operating mode.
• Adjust the set temperature.
• Wait a few minutes after restarting the unit.
• Check for error codes.

Do not handle the electronic board, capacitors, internal wiring, compressor, or refrigerant pipes yourself.

Electrical components may present a risk even when the system has been switched off.

When should you call an air conditioning technician?

Arrange a professional inspection if:

• The circuit breaker repeatedly trips.
• The indoor unit works but the outdoor unit does not.
• The system repeatedly tries to start and then stops.
• You hear unusual humming or mechanical noises.
• An error code appears.
• You notice a burning smell.
• The fault keeps returning.

A proper diagnosis makes it possible to identify the real cause of the problem before replacing unnecessary components.

Frequently Asked Questions

Why won’t my air conditioner turn on?

The cause may be a power supply issue, a tripped circuit breaker, a remote-control problem, or an electronic fault. Start by checking the power supply, batteries, settings, and electrical panel.

Why does my air conditioner turn on but not start?

This may be caused by incorrect settings, a compressor protection delay, or a fault in the outdoor unit. If the system still does not operate after several minutes, it should be inspected.

Why won’t the outdoor air conditioning unit start?

Possible causes include an electrical problem, a communication fault between the units, the electronic control board, the outdoor fan, or the compressor.

Why won’t the air conditioner compressor start?

The problem may be electrical, electronic, related to a faulty sensor, a damaged capacitor in certain systems, or a fault in the compressor itself. A technician should carry out the necessary tests to confirm the cause.

 

If your air conditioner won’t start, ClimateControl can inspect the system, identify the fault, and carry out the necessary repair to restore normal operation.