Common AC Problems and Troubleshooting Guide

Most AC problems follow recognisable patterns. A unit that stops cooling, starts making an unusual sound, or begins leaking water is telling you something specific about what has gone wrong. This guide covers the eight most common AC problems, what causes each one, and what to do, so you can diagnose the issue before calling a technician and avoid paying for a service call for something you can fix yourself.

Problem 1: AC Runs but Does Not Cool

This is the most frequently reported AC problem. The unit turns on, the fan blows, but the room temperature barely drops. The causes, in order of likelihood:

• Dirty filter. A heavily clogged filter restricts airflow to the point where the evaporator coil cannot exchange heat effectively. Clean the filter and allow the unit to run for 30 minutes before re-evaluating. This resolves the problem more often than any other cause.
• Wrong AC size for the room. An undersized AC will run continuously at full capacity and never bring a hot room down to target temperature. If the problem started with the unit from day one, sizing may be the cause. See what happens with an undersized AC.
• Low refrigerant. If the filter is clean and the unit was previously cooling well, low refrigerant is a common cause. Signs include ice forming on the indoor unit or refrigerant lines. Requires a technician to check pressure and locate the leak.
• Dirty evaporator or condenser coil. Coils coated in dust and grime cannot exchange heat at the designed rate. Annual professional servicing prevents this from becoming severe enough to affect cooling.
• Thermostat issue. Check that the unit is set to Cool mode (not Fan Only or Dry) and that the set temperature is below the current room temperature.

For a full diagnostic: why is my AC not cooling.

Problem 2: AC Will Not Turn On

• Remote battery dead. The most common cause. Replace the batteries before anything else.
• Circuit breaker tripped. Check the breaker box. An AC that repeatedly trips the breaker has an electrical fault that needs a technician.
• Power supply issue. Confirm the outlet is live and the AC power cable is fully connected.
• Capacitor failure. The capacitor starts the compressor and fan motors. A failed capacitor causes the unit to hum but not start. Requires a technician.
• Timer set accidentally. Check whether an off-timer or scheduled shutdown is active in the settings.

Problem 3: AC Making Unusual Noise

Full guide: AC making noise: what each sound means.

Problem 4: AC Leaking Water Indoors

Water dripping from the indoor unit is almost always caused by a blocked condensate drain line. The evaporator coil produces condensation during normal operation; this drains through a pipe to the outside or into a drain. When the drain line is partially blocked by algae, dust, or debris, water backs up into the drain pan and eventually overflows into the room.

Prevention: pour a cup of diluted white vinegar into the drain pan access point every three months to prevent algae growth. If the leak has already started, a technician can clear the drain line with compressed air or a wet-dry vacuum. Do not continue running the unit while water is leaking indoors, as it can damage ceilings and cause mould.

Other causes of indoor leaking: a cracked drain pan (replacement required), or a dirty evaporator coil that has iced up and is now defrosting. If ice is visible on the indoor unit, turn the AC off and let it defrost before investigating further.

Problem 5: AC Freezing Up

Ice on the indoor unit or refrigerant lines indicates that the evaporator coil is running colder than it should, causing moisture in the air to freeze on its surface. The two main causes:

• Restricted airflow. A clogged filter is the most common cause. Clean the filter, allow the ice to melt completely (switch to Fan Only mode to speed this up), then restart. If ice returns within a day, the coil itself may be dirty and need professional cleaning.
• Low refrigerant. Low refrigerant pressure causes the evaporator to run at an abnormally low temperature. Requires a technician to locate and repair the leak, then recharge.

Full guide: why is my AC freezing up.

Problem 6: AC Smells Musty

A musty or mouldy smell when the AC starts is almost always caused by mould or mildew growth on the evaporator coil, in the drain pan, or inside the air passages of the indoor unit. This happens when the unit is run in high-humidity conditions without adequate servicing, or when it sits idle for months before being restarted.

Prevention: run the unit in Fan Only mode for 30 minutes after each use to dry the coil. When restarting after a long idle period, be prepared for initial mustiness that may clear within an hour as the coil dries. If the smell persists, the coil needs professional cleaning with an appropriate fungicidal coil cleaner.

Full guide: why your AC smells musty and how to fix it.

Problem 7: AC Short Cycling

Short cycling means the compressor starts, runs briefly, and shuts off before the room reaches the set temperature, then restarts a few minutes later. This cycle repeats continuously. The main causes are an oversized AC (the most common in new installations), low refrigerant, or a refrigerant overcharge. An oversized AC cools the air near the thermostat sensor quickly, satisfies the thermostat, and shuts off before the whole room is properly cooled. The room warms again quickly and the cycle repeats.

If short cycling started suddenly in a unit that previously ran normally, the cause is more likely refrigerant-related or a thermostat fault. If it has been happening since installation, the unit is probably oversized for the room. See AC short cycling: causes and solutions.

Problem 8: High Electricity Bill Without Obvious Cause

If your electricity bill has risen significantly without any change in usage pattern, the AC is the most likely culprit. Common causes include a dirty filter (adds 10 to 25 percent), dirty coils (adds 10 to 30 percent), low refrigerant (adds 5 to 20 percent), and general efficiency degradation from skipped servicing. An annual service typically restores the unit to near its rated efficiency and the bill reduction is often recovered within one to two months of the hot season.

Frequently Asked Questions

Sources and Further Reading

• Bureau of Energy Efficiency, AC maintenance and troubleshooting (beeindia.gov.in)
• Air-Conditioning, Heating and Refrigeration Institute, diagnostic guidelines (ahrinet.org)
• U.S. Department of Energy, room air conditioner problem-solving (energy.gov)