10 Signs Your Air Conditioning Needs Repair

Introduction: Do Not Ignore the Warning Signs

Your air conditioning system is designed to run reliably for many years, but like any mechanical system, components can wear out, refrigerant levels can drop, and electrical faults can develop. The good news is that most AC problems give you warning signs before they become major failures. Catching these signs early and calling a qualified engineer can save you from expensive emergency repairs, uncomfortable downtime in the middle of a heatwave, and even premature system replacement.

At London AC Pro, we are F-Gas certified AC repair specialists covering all 32 London boroughs. We repair all major brands including Daikin, Mitsubishi Electric, Samsung, LG, Fujitsu and Toshiba. Here are the 10 most common signs that your air conditioning needs repair — and what to do about each one.

1. Weak or Reduced Airflow

If you notice that the air coming from your indoor unit feels weaker than usual, there are several possible causes. The most common is a dirty or clogged air filter, which restricts airflow. This is a maintenance issue you can often resolve yourself by cleaning or replacing the filter. However, weak airflow can also indicate a failing indoor fan motor, a blocked evaporator coil, or damaged ductwork (in ducted systems).

What to do: Clean or replace your air filters first. If the problem persists, call a professional. Do not continue running the system with restricted airflow, as this puts strain on the compressor and can cause further damage.

2. Warm Air Coming from the Unit

If your AC is running in cooling mode but blowing warm or room-temperature air instead of cold air, something is wrong. Common causes include low refrigerant levels (often due to a leak), a faulty compressor, a failed reversing valve, a dirty outdoor coil blocking heat rejection, or an electrical fault preventing the outdoor unit from running.

What to do: Check that the system is set to cooling mode and that the set temperature is below the current room temperature. Check that the outdoor unit is running. If everything looks correct but the air is still warm, call London AC Pro for a diagnostic visit.

3. Strange or Unusual Noises

Air conditioning systems should run quietly. Modern wall-mounted units from Daikin, Mitsubishi and Samsung operate at just 19–22 dB(A) — barely audible. If you hear unusual sounds, pay attention to the type of noise:

• Rattling or vibrating — loose panels, screws, or a foreign object in the unit. The outdoor unit may need re-levelling or anti-vibration mounts.
• Grinding or scraping — a failing fan motor bearing or a fan blade hitting something. Stop the unit and call an engineer.
• Hissing or bubbling — a refrigerant leak. This requires urgent professional attention as refrigerant is a controlled substance under F-Gas regulations.
• Clicking — a relay or contactor cycling. Some clicking during start-up is normal, but continuous clicking suggests an electrical fault.
• Squealing or screeching — a slipping or worn fan belt (in older or commercial systems) or a failing motor.

What to do: Do not ignore unusual noises. Turn off the system if the noise is severe (grinding, scraping) and call us on 07301 010244.

4. Unpleasant Odours

If your AC produces a musty, mouldy or stale smell when it starts up, this usually indicates biological growth — mould, mildew or bacteria — on the indoor evaporator coil or in the drainage tray. This is a health concern as well as a comfort issue, as the contaminated air is circulated throughout the room.

A burning or electrical smell is more serious and may indicate overheating wiring, a failing motor or an electrical component burning out. Turn off the system immediately and call an engineer.

What to do: A musty smell is resolved by a professional deep clean of the indoor unit, including antibacterial coil treatment. A burning smell requires immediate professional attention. Visit our AC repair page for details.

5. Water Leaks from the Indoor Unit

If you see water dripping from the indoor unit or pooling on the wall or floor beneath it, the most likely cause is a blocked condensate drain. During normal operation, the AC produces condensation (water) which drains away through a pipe or pump. If the drain becomes blocked by dirt, algae or debris, the water backs up and overflows from the unit.

Other possible causes include a cracked or disconnected drain pipe, a failed condensate pump, or excessive condensation due to very high humidity levels.

What to do: Turn off the system to prevent further water damage. Place towels or a container to catch drips. Call a professional to clear the blockage and check the drain system. This is usually a quick and inexpensive repair.

6. Short Cycling (Frequent On-Off Switching)

Short cycling is when the compressor starts, runs for a very short time (a few minutes or less), stops, and then starts again repeatedly. This is hard on the compressor and dramatically increases energy consumption. Common causes include an oversized system, low refrigerant charge, a dirty condenser coil restricting heat rejection, a faulty thermostat, or an electrical issue.

What to do: Short cycling should be investigated by a professional. Continuing to run a short-cycling system risks compressor failure, which is the most expensive component to replace.

7. Ice or Frost on the Indoor Unit or Pipes

If you see ice forming on the indoor unit's evaporator coil or on the refrigerant pipes, this indicates a problem with airflow or refrigerant charge. The most common causes are a dirty air filter restricting airflow across the evaporator, or low refrigerant levels causing the coil temperature to drop below freezing. A faulty expansion valve or fan motor can also cause icing.

What to do: Turn off the system and allow the ice to melt naturally (do not scrape it off). Check and clean the air filters. If the problem recurs after cleaning the filters, call a professional to check the refrigerant charge and system components.

8. Higher Energy Bills Than Expected

If your electricity bills have increased noticeably without a corresponding increase in usage, your AC may be running inefficiently. Common causes include dirty filters and coils reducing heat transfer, low refrigerant forcing the compressor to work harder, a failing compressor losing efficiency, or a faulty thermostat causing the system to run longer than necessary.

What to do: Book a professional service. A thorough clean, refrigerant check and system inspection will identify the cause and restore efficiency. The cost of a service is typically far less than the ongoing cost of running an inefficient system.

9. Error Codes or Flashing Lights

Modern AC units from Daikin, Mitsubishi, Samsung, LG, Fujitsu and Toshiba have sophisticated self-diagnostic systems. When the system detects a fault, it displays an error code on the indoor unit's display or flashes indicator lights in a specific pattern. Each code corresponds to a specific fault — communication error, sensor failure, refrigerant pressure anomaly, compressor protection, and so on.

What to do: Note the error code or flashing pattern and call London AC Pro. Our engineers are factory-trained on all major brands and can diagnose and resolve error codes quickly. Do not attempt to reset the system repeatedly, as this may mask an underlying fault.

10. The System Will Not Turn On at All

If your AC does not respond to the remote control or thermostat at all, start with the basics: check that the batteries in the remote control are fresh, check that the system is switched on at the isolator, and check that the circuit breaker has not tripped. If everything looks fine but the system still will not start, the issue may be a blown fuse, a faulty PCB (circuit board), a failed compressor, or a wiring fault.

What to do: Check batteries and isolators first. If the circuit breaker has tripped, reset it once — if it trips again immediately, do not reset it again (this indicates a fault). Call a professional for diagnosis.

When to DIY vs When to Call a Professional

Some minor maintenance tasks are safe and appropriate for homeowners to do themselves:

• DIY: cleaning air filters, clearing debris from around the outdoor unit, replacing remote control batteries, wiping down the indoor unit exterior
• Call a professional: anything involving refrigerant, electrical components, error codes, water leaks, unusual noises, or any situation where you are unsure

Never attempt to top up or handle refrigerant yourself. Under F-Gas regulations, only certified engineers are legally permitted to work with fluorinated refrigerant gases.

Prevention Is Better Than Cure

The best way to avoid AC repairs is to invest in regular preventive maintenance. An annual service (or twice-yearly for commercial systems) catches small issues before they become big problems, keeps your system running efficiently, and extends its lifespan. Read our detailed guide: How Often Should You Service Your Air Conditioning?

London AC Pro: Fast, Reliable AC Repair Across London

If your air conditioning is showing any of these warning signs, do not delay — call London AC Pro for a fast, professional diagnosis and repair. We are F-Gas certified, fully insured, and carry a comprehensive stock of spare parts for all major brands. We offer same-day and next-day emergency callouts across all 32 London boroughs. Call us on 07301 010244 or book a repair online. Keep your home or business cool and comfortable — we are here to help.