Air Conditioner Not Working?

When air conditioning stops cooling, blows warm air or will not turn on, the cause is often simple: a setting, a timer, a dirty filter or a tripped switch. This guide takes you through the checks you can safely make yourself, the faults that need an engineer and what to note before you call us.

Switch It Off and Call Us If

• you notice a burning or electrical smell

• the unit makes a loud grinding, screeching or banging noise

• water is dripping onto electrics

• the circuit breaker trips again after you reset it

Switch the system off at the controller and, if it is safe to do so, at its isolator switch, then call us on 01772 288651.

If It Will Not Turn On

1. Check the remote. Replace the batteries and point it straight at the indoor unit.

2. Check the power. Make sure the isolator switch, usually next to the outdoor unit, is on and the circuit breaker in your consumer unit has not tripped.

3. Check the timer. A timer or a schedule, including one set in an app, may be keeping the unit off.

4. Wait a few minutes. Many systems wait a few minutes before restarting after being switched off, to protect the compressor.

If a light is flashing on the indoor unit, or the controller shows an error code, note it down. It tells an engineer what the system has detected.

If It Runs but Does Not Cool

• Check the mode. Make sure it is on cool, not fan, dry or heat, and that the set temperature is below the room temperature. See air conditioning remote symbols.

• Give it time. On a hot day, a room that has been shut up in the sun can take a while to come down.

• Clean the filters. Blocked filters cut the airflow and the cooling. See how to clean your air conditioner.

• Check the outdoor unit. It needs a clear flow of air, so clear away leaves, plants or anything stacked against it.

• Close windows and doors. Cooling a room with the windows open lets the heat straight back in.

• Check the size. If the room has never cooled properly, the unit may be too small for it. See what size air conditioner you need.

If It Blows Warm Air

• Wrong mode. Heat mode, and sometimes auto mode, blows warm air. Switch to cool.

• Defrost in winter. In heating mode, the indoor fan may stop, or the air may feel cooler, for a few minutes while the outdoor unit defrosts. This is normal.

• Outdoor unit not running. If the indoor fan runs but the outdoor unit is silent, the compressor or outdoor fan may have a fault, or a sensor may have stopped it.

• Low refrigerant. Refrigerant does not get used up, so low refrigerant means there is a leak. An engineer needs to find and repair it.

If There Is Ice on the Unit or the Pipes

Ice on the indoor coil or the pipework is usually caused by low airflow from blocked filters, or by low refrigerant. Switch the system off, let the ice melt, clean the filters and try again. If the ice comes back, call an engineer. Ice on the outdoor unit in heating mode is normal in cold weather, and the defrost cycle clears it.

If It Is Noisy

• A rattle can come from a loose panel or something touching the unit.

• Some clicking as the system starts and stops is normal.

• A gurgling or bubbling sound is usually refrigerant or water moving through the pipes.

• Grinding, screeching or loud banging is not normal. Switch the system off and call us.

What Not to Do

• Do not open the casing or touch the electrics.

• Do not keep resetting a circuit breaker that trips.

• Do not chip ice off the coil or the pipes.

• Do not let anyone without the right qualifications work on the refrigerant circuit. On most systems, the law requires an F-Gas certified engineer.

Book a Repair

If these checks do not solve it, call 01772 288651 or use the form below. Tell us what the system is doing, any error code or flashing light, and roughly how old it is. Our F-Gas certified engineers will find the fault and explain your options.

If water is leaking from the indoor unit, see air conditioner leaking water. Regular servicing catches many faults early: see our air conditioning servicing page and the top signs your AC system needs repair. If an old system keeps breaking down, see replacing old air conditioning.

Frequently Asked Questions

Why does my air conditioner turn on and off on its own?

A timer, a schedule in the app or a sleep setting may be switching it. Frequent short cycles can also be a sign that the unit is too large for the room.

Why is my air conditioner cooling some rooms but not others?

Each indoor unit cools its own room. In a multi-split system, check that each unit is switched on and that all the units are set to the same mode. Most multi-split systems cannot heat one room and cool another at the same time. If one unit still will not cool, call us.

Last updated: November 2026.