Air Conditioning Problems: How to Diagnose and Fix Common Issues

When your air conditioning unit breaks down, it's not only frustrating — it raises immediate questions. How much will a repair cost? Do I need a new unit, or just a replacement part? How long will I be without it? In most cases, the root cause is poor or absent maintenance, and many issues can be resolved without calling an engineer. This guide covers the most common air conditioning problems and what to do about them.

1. Low Refrigerant Levels

Low refrigerant is one of the most common causes of air conditioning failure. Refrigerant is the substance that makes cooling possible, and if levels drop — usually due to a leak in the system — the unit will either perform poorly or stop cooling altogether.

If you suspect a refrigerant leak, contact a qualified F-Gas registered engineer immediately. Refrigerant gases are environmentally harmful and it is illegal for unqualified persons to handle them. To help locate a leak before calling an engineer, a refrigerant leak detector spray or an electronic detector such as the Mastercool 55800 Intellasense can help pinpoint the source. Of all common AC problems, refrigerant leaks are typically the most costly to rectify.

2. Frozen Evaporator Coils

The evaporator coil cools the refrigerant as part of the cooling cycle. If airflow over the coil is restricted, it can freeze, causing the unit to stop cooling effectively or shut down entirely. The most common cause is a dirty or blocked filter, though low refrigerant can also contribute.

The fix is straightforward: turn the unit off and allow the coil to defrost fully (this can take several hours), then clean or replace the filter before restarting. To prevent recurrence, clean filters monthly and keep the area around the indoor unit clear. Regular cleaning of the evaporator coil itself with a purpose-made cleaner such as Coil Care Antimicrobial Evaporator Cleaner or EasyClean Evaporator & Condenser Coil Cleaner will also help maintain airflow and prevent bacterial build-up.

Air conditioners that receive regular maintenance last approximately 40% longer than those that don't — a significant saving over the lifetime of the unit.

3. Clogged Condensate Drain

Air conditioning systems remove moisture from the air as part of the cooling process. This condensate drains away through a pipe — but over time, algae and debris can cause blockages. A blocked drain can cause water to back up, leading to water damage, dampness, and eventually a system shutdown via the safety float switch.

For minor blockages, flushing the drain line with a 1:1 mix of white vinegar and warm water is effective. For more stubborn blockages — particularly in systems with condensate pumps — a purpose-made drain treatment gel is more reliable. Regular treatment every few months will prevent blockages from developing in the first place.

4. Thermostat Issues

A faulty thermostat can cause the unit to run intermittently, fail to reach the set temperature, or waste energy running when it isn't needed. There are no obvious visual signs of a faulty thermostat — if your unit is behaving erratically and other causes have been ruled out, the thermostat is worth investigating.

Older analogue dial-style thermostats are more prone to failure than modern digital or smart thermostats. If replacement is needed, here's how to do it safely:

How to Replace an AC Thermostat

1. Turn off the power. Switch off the circuit breaker for your AC unit. As an additional precaution, disconnect the main power leads from the control board or transformer.
2. Remove the old thermostat. Remove the cover plate and unscrew the thermostat from the wall. Before disconnecting any wires, photograph the wiring connections clearly — this is essential for correct reinstallation.
3. Install the new thermostat. Connect each wire to the correct terminal on the new thermostat, referring to your photos. Mount the unit to the wall, ensuring it is level and secure.
4. Test the installation. Restore power and set the thermostat to your desired temperature. You should hear the system start up and begin conditioning the space within a few minutes.

Important: Always check that the replacement thermostat is compatible with your specific HVAC system before purchasing. Note the make and model of your existing unit and check the product specifications carefully. If in doubt, consult a qualified HVAC technician.

5. Faulty Fuse or Tripped Circuit Breaker

Sometimes the cause of a non-responsive unit is as simple as a blown fuse or tripped breaker. Check your consumer unit first before assuming a more serious fault. If the breaker trips repeatedly after being reset, there is an underlying electrical fault that requires an electrician — do not continue resetting it.

6. Remote Control Not Working

Before assuming the unit itself is faulty, check the remote control. Common remote control issues and fixes:

• Distance and line of sight: Most remotes require a clear, unobstructed line of sight to the unit's sensor, within approximately 6 metres. Move closer and ensure nothing is blocking the signal path.
• Dirty sensors: Clean the IR sensor on both the remote and the indoor unit with a dry cloth. Dust and grime can block the signal.
• Batteries: Replace the batteries — even if the display is still showing, weak batteries can prevent the signal from reaching the unit. Ensure batteries are inserted the correct way round, and clean the battery contacts with a dry cotton bud if there is any corrosion or debris.
• Wrong settings: Check you are pressing the correct buttons for the function you want. Refer to the user manual if needed.
• Damaged IR LED: If the remote has been dropped or sat on, the IR LED at the tip may be misaligned or damaged. You can test this by pointing the remote at a smartphone camera and pressing a button — a working IR LED will appear as a flashing light on the camera screen.

If the remote is beyond repair, a universal replacement remote control compatible with Daikin, Samsung, LG and many other brands is a cost-effective solution. The Mimic AC Remote Controller is another option that can learn and replicate the signals from most existing remotes.

Frequently Asked Questions

Why is my air conditioner blowing warm air?

The most likely causes are low refrigerant, a frozen evaporator coil, or a dirty filter restricting airflow. Check and clean the filter first, then call an engineer if the problem persists.

Why does my air con keep turning itself off?

This is often caused by a blocked condensate drain triggering the safety float switch, a faulty thermostat, or an electrical issue. Check the drain line and filter first.

Can I fix a refrigerant leak myself?

No. Handling refrigerant requires an F-Gas qualification. Contact a registered engineer. You can use a leak detector to help locate the source before they arrive.

How do I know if my thermostat needs replacing?

If your unit runs erratically, fails to reach the set temperature, or runs continuously without cycling off, and other causes have been ruled out, the thermostat may be at fault. An HVAC technician can confirm this with a simple test.