Common Air Conditioner Problems and How to Troubleshoot Them

Nothing is more frustrating than an air conditioner that stops working properly—especially during a heatwave. While some problems require professional repair, many common issues have simple causes that you can diagnose and sometimes fix yourself. This troubleshooting guide covers the most frequent air conditioner problems, their likely causes, and when you should attempt a DIY fix versus calling a technician.

Problem 1: AC Not Turning On

You press the button, but nothing happens—no lights, no fan, no response at all.

Possible Causes and Solutions

• Power issues: Check the power point is switched on, the circuit breaker hasn't tripped, and the AC's isolator switch is on. Reset any tripped breakers and try again.
• Remote control: Replace the batteries. Try the buttons on the indoor unit itself if available.
• Thermostat setting: Ensure the set temperature is below (for cooling) or above (for heating) the current room temperature.
• Timer function: Check if a timer is active that's preventing operation. Reset to cancel any timer functions.
• Safety lockout: After a power outage, some units have a delay before restarting. Wait 5 minutes and try again.

When to call a pro: If power is definitely reaching the unit but it won't respond, internal electrical components may have failed and require professional diagnosis.

Problem 2: AC Running But Not Cooling

The fan is blowing, but the air isn't cold—one of the most common complaints in summer.

Possible Causes and Solutions

• Dirty filters: The number one cause. Clean or replace filters immediately. This alone fixes the problem in many cases.
• Incorrect mode: Ensure the unit is set to "Cool" mode, not "Fan" or "Heat." Check that the set temperature is lower than room temperature.
• Blocked outdoor unit: Check for debris, overgrown plants, or obstructions blocking airflow around the outdoor condenser.
• Dirty coils: Both indoor and outdoor coils need to be clean. Outdoor coils can be gently hosed; indoor coils typically need professional cleaning.

When to call a pro: If filters are clean and outdoor unit is clear but cooling is still weak, you may have a refrigerant leak, compressor issue, or other problem requiring professional repair.

Problem 3: Water Leaking from Indoor Unit

Water dripping from your wall-mounted indoor unit is concerning but often has a simple cause.

Possible Causes and Solutions

• Blocked condensate drain: The most common cause. The drain line can become clogged with dust, mould, or algae. Clear the external drain opening if accessible, or pour a cup of white vinegar through the drain from inside the unit.
• Dirty or frozen evaporator coil: Restricted airflow from dirty filters can cause the coil to ice up, then drip excessively when it thaws. Clean filters and let ice melt naturally.
• Tilted indoor unit: Units should slope slightly toward the drain side. If knocked or incorrectly installed, water pools and overflows.
• Cracked drain pan: Over time, the internal drain pan can crack, causing leaks. This requires professional replacement.

When to call a pro: If clearing the drain doesn't solve the issue, or if the leak is near electrical components, professional inspection is needed to prevent water damage and electrical hazards.

Problem 4: Strange Sounds

Air conditioners should operate quietly. Unusual noises often indicate problems.

Diagnosing Sounds

• Rattling or vibrating: Often a loose panel, screw, or component. Check all covers are secure. May also indicate debris in the outdoor unit.
• Squealing or screeching: Typically a belt or bearing issue in the fan motor. Turn off and call a technician.
• Grinding: Suggests a serious motor problem. Stop using immediately and get professional help.
• Clicking (constant): Could indicate a relay problem or electrical issue. Occasional clicking when starting/stopping is normal.
• Bubbling or gurgling: May indicate refrigerant issues or a blocked drain line.
• Hissing: Could be normal refrigerant flow, but persistent hissing may indicate a refrigerant leak.

When to call a pro: Any grinding, squealing, or persistent unusual noises warrant professional inspection. Continuing to operate with mechanical problems often causes more extensive damage.

Problem 5: Bad Smells

Unpleasant odours from your air conditioner can have several causes, some concerning.

Identifying and Addressing Odours

• Musty or mouldy smell: Indicates mould or mildew growth in the unit, ductwork, or filters. Clean filters, run the "dry" mode for an hour to reduce moisture, and consider professional coil cleaning.
• Burning smell: Turn off immediately. Could be electrical components overheating. Do not use until professionally inspected.
• Chemical or sweet smell: May indicate refrigerant leak. Turn off, ventilate the room, and call a technician.
• Rotten egg smell: Could indicate a dead animal in the outdoor unit or ductwork. Requires inspection and removal.
• Cigarette or stale odours: Filters absorb odours over time. Replace filters and consider a deep clean.

Problem 6: AC Cycling On and Off Frequently

"Short cycling" means the unit starts, runs briefly, then shuts off—repeating this pattern without properly cooling.

Possible Causes and Solutions

• Dirty filters: Restricts airflow, causing the system to overheat and shut down. Clean filters first.
• Oversized unit: A unit too large for the space cools quickly, shuts off, then temperature rises and it restarts. This is a sizing issue often only resolved by replacement.
• Thermostat issues: Faulty thermostat or one placed in a hot spot can trigger erratic cycling.
• Low refrigerant: Causes pressure problems that trigger safety shutoffs.
• Blocked outdoor unit: Overheating from restricted airflow triggers safety cutouts.

When to call a pro: If filters are clean and outdoor unit is clear, professional diagnosis is needed to check refrigerant levels, electrical components, and thermostat function.

Problem 7: Ice on the Unit

Ice forming on refrigerant lines, the evaporator coil, or the outdoor unit indicates a problem.

Possible Causes and Solutions

• Dirty filters: The most common cause of indoor coil icing. Clean filters and let ice melt naturally (may take several hours).
• Low refrigerant: Reduces system pressure and causes freezing. Requires professional repair.
• Blocked airflow: Ensure vents aren't covered and airflow isn't restricted.
• Faulty blower fan: If the indoor fan isn't moving enough air, the coil can freeze.
• Running in cold weather: Most split systems shouldn't run in cooling mode when outdoor temperatures are below about 15°C.

When to call a pro: If ice persists after cleaning filters and ensuring good airflow, you likely have a refrigerant or mechanical issue requiring professional repair.

When to Replace vs Repair

Sometimes repair isn't cost-effective. Consider replacement if:

• Your unit is over 10-15 years old
• Repairs would cost more than 50% of a new unit
• You've had multiple repairs in recent years
• The system uses R22 refrigerant (now phased out)
• Energy bills have increased significantly over time
• Modern efficiency improvements would substantially cut running costs

Preventing Future Problems

Regular maintenance prevents most common problems:

• Clean filters every 2-4 weeks during heavy use
• Check outdoor unit clearance monthly
• Schedule annual professional servicing
• Address small issues before they become major repairs
• Keep the condensate drain clear

For detailed maintenance procedures, see our comprehensive AC maintenance guide.