AC not cooling? The common faults and what each one usually means

An AC that is running but not cooling is almost always one of six things: airflow, refrigerant, the outdoor unit not doing its job, the electricals that start it, the controls, or simply the wrong setting. You can narrow it down in a few minutes by matching what you see and hear to the list below. That tells you whether it is a filter you can wash, or a fault that needs a technician and which part he is likely to bring.

The order below runs from the most common and cheapest to the least common and most expensive. Work down it in that order.

Weak airflow, room cools slowly

If the air from the indoor unit is cold but thin, the first suspect is a blocked filter. Slide the front panel up, pull the two mesh filters out, and hold them to the light. In a Nellore summer, with windows open in the evening and dust from the road, a filter can choke in three or four weeks.

If the filters are clean and airflow is still weak, the indoor coil behind them is probably clogged with fine dust and the mould that grows on it in humid months. That needs a proper coil wash, not a brush. A service at ₹399 covers the coil, filters and drain.

A blower wheel coated in grime, or a fan motor that has slowed, produces the same symptom and is found in the same service.

Air is not cold at all, outdoor unit is running

Check the remote first: the mode should show the snowflake (Cool), not the fan, the drop (Dry) or the sun. Set the temperature to 24°C and wait ten minutes. You would be surprised how often this is the whole fault.

If the setting is right and the outdoor fan is turning but the air stays warm, the likely causes are:

• Low refrigerant. Cooling fades over weeks rather than stopping overnight; the thin copper pipe at the outdoor unit may be frosted or sweating heavily. Refrigerant does not get consumed, so low gas means a leak, and the repair is to find and fix the leak, then recharge. A refill on a split unit is ₹700 on the rate card.
• Blocked outdoor coil. If the fins on the outdoor unit are matted with dust, cotton fluff or, near the coast, a white crust of corrosion, the unit cannot dump heat. It will cool poorly and run hot. Washing the outdoor coil is part of a service.
• Compressor not running. The fan spins but the compressor inside is silent or only hums. See the next section.

Outdoor unit hums, clicks or does not start

Stand near the outdoor unit when the AC is switched on. You should hear the fan start and, a few seconds later, a heavier note as the compressor kicks in.

• Hum for a few seconds, then silence, then the hum again. The compressor is trying to start and cannot. The usual cause is a failed or weak capacitor, a small cylindrical part that takes a beating in 40°C afternoons. It is cheap relative to anything else on this page, charged at MRP and shown to you before fitting.
• Nothing from the outdoor unit at all. Could be the contactor or relay, a tripped thermal cut-out, a wiring fault between indoor and outdoor, or the PCB not sending the signal. A diagnosis visit is ₹350 and covers the standard labour.
• Starts, then the MCB trips. Stop. Do not keep resetting it. That is an electrical fault or a compressor drawing too much current, and repeated resets can turn a repairable fault into a dead compressor.

Evening voltage dips make all of this worse. A compressor that starts fine at noon may fail to start at 7 pm when the supply sags. If the problem only shows in the evening, mention that when booking.

Indoor unit does not respond or blinks

If the display is dead, check the plug, the switch and the MCB. If the unit is powered but ignores the remote, change the remote batteries before anything else.

A blinking light or an error code on the display usually points to a sensor, a communication fault between the two units, or the PCB. Write down the code or count the blinks; it saves the technician time. PCB repair versus replacement is a judgement call that depends on the board, and the part is shown to you with its price before any decision.

Water dripping from the indoor unit

Water is not a cooling fault, but people often notice it first. It almost always means the drain line is blocked with slime or, in hard-water areas, scale, or the unit has lost its slight tilt toward the drain. A drain flush is in every service. If water drips only when it is very humid, the insulation on the pipes may have perished and be sweating.

The expensive one

A compressor that has genuinely failed, with correct gas and good voltage, is the one fault where you should pause and think about the unit's age before approving the work. Our repair-or-replace guide goes through that decision. Everything else above is routine and worth fixing.

What to tell the technician

When you book, mention three things: what the air feels like (cold but weak, or not cold), what the outdoor unit is doing (running, humming, silent, tripping), and when it started (sudden or gradual). Those three answers point to the cause more often than not and mean the technician arrives with the right part in the bag.