AC Not Cooling? 10 Common Causes and How to Fix Them

Your AC is running, the fan is blowing and the remote shows 18°C. Yet the room still feels warm. This is one of the most common problems we see at Urban Appliances Care, especially in Kolkata’s humid summers and long monsoon months.

An AC that is not cooling properly usually has one of 10 causes: dirty filters, wrong settings, low refrigerant (gas), dirty coils, a failed capacitor, a faulty compressor, a frozen coil, a blocked outdoor unit, a faulty sensor or PCB, or a mismatch between the AC and the room. Some can be fixed in minutes at home. Others need a trained technician.

This guide explains each cause, how to spot it and what to do next, so you don’t waste money on the wrong repair.

Quick Checks Before You Call a Technician

Spend two minutes on these first. They solve a surprising number of cases.

• Confirm the AC is in Cool mode, not Fan, Dry or Auto, with the temperature set well below room temperature.
• Check that all windows and doors are closed.
• Look at the indoor unit for a visible layer of dust, ice or water dripping.
• Make sure the outdoor unit is running and its fan is spinning.
• Check whether the MCB or stabilizer has tripped.

If none of these help, read on.

10 Reasons Your AC Is Not Cooling (and How to Fix Each)

Most AC cooling problems come down to a handful of causes, ranging from a simple dirty filter to a gas leak or a failed part. Go through the reasons below in order, starting with the easiest to check, to find out what is affecting your AC.

1. Dirty or Clogged Air Filters

Symptoms: weak airflow, slow cooling, a musty smell, the AC running for hours without reaching the set temperature.

Filters trap dust before it reaches the cooling coil. When they clog, airflow drops and the room warms up. In a dusty city like Kolkata, filters can clog faster than you’d expect.

Fix: Switch off the AC, remove the filters and rinse them gently with water. Dry them fully before refitting. Do this every 2–4 weeks during heavy use. If cooling is still poor, the problem is deeper than the filter.

2. Incorrect Settings or Mode

Symptoms: air feels cool but not cold, or the AC cools briefly and then blows normal air.

Dry mode reduces humidity without cooling as strongly. Fan mode doesn’t cool at all. A thermostat set near room temperature also stops the compressor early.

Fix: Set the AC to Cool mode, lower the temperature, and set the fan to auto or medium. If the AC has an energy-saving or sleep mode, turn it off while testing.

3. Low Refrigerant (AC Gas Leakage)

Symptoms: The AC runs, but the air is only slightly cool, ice forms on the copper pipe or outdoor unit, you hear a hissing sound, or cooling gets worse over weeks.

Refrigerant carries heat out of your room. Your AC doesn’t consume gas the way a car consumes fuel, so low gas almost always means a leak. Simply topping up without fixing the leak means the problem comes back.

Fix: This needs a technician. They locate and repair the leak, then refill the correct gas (R22, R32 or R410A, depending on your model). To understand what refilling involves and what affects the price, read our guide on AC gas refilling cost in Kolkata.

4. Dirty Evaporator or Condenser Coils

Symptoms: reduced cooling, higher electricity bills, the AC taking much longer to cool, a visible layer of dust or grime on the coils or outdoor unit.

The indoor (evaporator) coil absorbs heat from your room and the outdoor (condenser) coil releases it. Dirt on either coil acts like a blanket, and the AC struggles to cool.

Fix: A basic filter wash won’t reach these coils. A professional jet or deep-cleaning service removes the dirt properly, without damaging the delicate fins. Our AC repair and service team in Kolkata handles cleaning for split, window and cassette units.

5. Faulty Capacitor

Symptoms: the indoor fan runs but the outdoor unit doesn’t start, a humming sound from the outdoor unit, the compressor or outdoor fan not spinning, frequent tripping.

The capacitor gives the compressor and fan motor the electrical push to start. When it weakens, the motor struggles or won’t start, so the AC blows room-temperature air.

Fix: capacitor replacement is a quick repair for a technician but not safe for DIY, as capacitors can hold a charge even when the power is off. If you have a window AC, our guide to window AC parts shows where the capacitor sits and what it does.

6. Compressor Problems

Symptoms: no cooling at all, loud rattling or grinding from the outdoor unit, the outdoor unit overheating, the AC tripping the MCB repeatedly.

The compressor is the heart of the AC. It circulates refrigerant through the system. Overheating, electrical faults or long-term neglect can damage it.

Fix: Switch the AC off if you hear unusual noises, and call a technician. Many “compressor problems” turn out to be a capacitor or relay fault, which costs far less than a compressor replacement. A proper diagnosis prevents an unnecessary expense.

7. Frozen Evaporator Coil or Choked Drain

Symptoms: ice on the indoor coil or the pipe, water dripping from the indoor unit, very weak airflow.

Low airflow (from dirty filters or a blocked blower) or low refrigerant can make the coil too cold and freeze. A frozen coil can’t absorb heat, so the AC stops cooling effectively.

Fix: switch the AC to Fan mode for a few hours to defrost it. Then clean the filters. If it freezes again, the cause is likely a gas leak or airflow fault that needs professional repair.

8. Blocked or Faulty Outdoor Unit

Symptoms: the outdoor unit is hot, noisy or silent, the outdoor fan doesn’t spin, the AC cools, then stops.

The outdoor unit must release heat freely. A blocked unit (by dust, a wall, stored items or bird nests) or a failed fan motor means heat stays trapped in the system.

Fix: clear at least a few feet of space around the outdoor unit and remove visible debris. If the fan still doesn’t spin, the fan motor or capacitor may need replacing.

9. Faulty Thermostat, Sensor or PCB

Symptoms: The AC cools unpredictably, switches off too early, never starts the compressor, or shows an error code.

The room-temperature sensor tells the AC when to cool and when to stop. If it’s faulty or misplaced, the AC receives wrong information. On inverter ACs, a PCB (control board) fault can also affect compressor operation.

Fix: This is an electronic fault and needs technician diagnosis. Do not open the control board yourself.

10. Wrong AC Capacity or Poor Installation

Symptoms: a new AC that never cools well, one room cools but a larger or sun-facing room doesn’t, uneven cooling near windows.

An undersized AC can’t keep up with the room’s heat load. Poor installation, such as long pipe runs, bent copper pipes or a badly placed indoor unit, also reduces efficiency. Top-floor rooms, west-facing walls and rooms with big windows need more cooling capacity.

Fix: A technician can check the installation and pipe layout. If the AC is simply too small for the room, the fix is a lighter heat load (curtains, sealing gaps) or a larger AC.

DIY Fix or Call a Technician?

You can safely do these yourself:

• Clean the filters
• Check the mode and temperature settings
• Clear debris around the outdoor unit
• Reset the AC by turning it off for a few minutes

Call a technician for these:

• Gas leakage or refilling
• Capacitor, compressor, PCB or sensor faults
• Repeated MCB tripping
• Ice that keeps coming back
• Any burning smell, sparks or loud noises

Never try to open the outdoor unit, handle refrigerant or touch electrical components yourself. It’s unsafe and can turn a small repair into an expensive one.

How to Stop Your AC from Losing Cooling Again

• Clean the filters every few weeks in peak season.
• Get a full service before summer begins, not after the first breakdown.
• Keep the outdoor unit clear and shaded from debris.
• Don’t ignore small signs such as odd sounds, a longer cooling time or slightly higher bills.
• Consider an annual maintenance contract if you have multiple ACs. It catches problems early.