AC not cooling? Here is how to find out why

An AC that runs but does not cool is one of the most frustrating problems during a Lahore summer. The unit is on, the fan is blowing air, but the temperature in the room barely drops. Before you call a technician, there is some basic checking you can do that may pinpoint the problem. And if the fix needs a professional, knowing what to look for helps you avoid unnecessary repairs.

This guide walks through the common reasons a split or inverter AC stops cooling properly — from the simplest fix you can do yourself to the more complex faults that need a technician with a gauge set and electrical tester.

Check the air filter first

The most common cause of weak cooling is a clogged air filter. When the filter is blocked with dust, airflow across the indoor coil drops significantly, and the unit cannot transfer heat from the room air to the refrigerant. The result: the AC runs for hours but the room stays warm. Cleaning or replacing the filter takes five minutes and solves the problem if this is the only issue. On most split ACs, the filter is behind the front panel, accessible by sliding it out and rinsing under running water. Let it dry completely before reinserting.

If cleaning the filter does not help, the problem is likely elsewhere, and a technician visit will be needed.

Check the condenser coil outside

The outdoor unit — the part that sits on the ground or mounted on a wall bracket — needs airflow across its coils to release the heat it removes from your room. If the fins are clogged with dust, dry leaves, or fluff from nearby trees, the heat cannot escape and the AC cannot cool effectively. This is especially common in homes where the outdoor unit is placed near a garden or a dusty roadside. Hosing down the outdoor coil carefully can restore performance, but take care not to bend the aluminium fins. A damaged coil may need professional cleaning or straightening.

Check the thermostat and settings

It sounds obvious, but the thermostat setting is often overlooked. If the unit is set to fan-only mode rather than cool mode, it will blow air but not actually cool. Check that the mode is set correctly and the set temperature is at least a few degrees below the current room temperature. On inverter ACs, the unit runs at variable speed, so it may take longer to reach the set temperature than a conventional unit, especially on a very hot day. Patience for 15 to 20 minutes after switching on is normal before expecting full cooling.

Low refrigerant charge

If the filter is clean, the outdoor coil is clear, and the settings are correct, the next likely cause is a low refrigerant charge — commonly called "low gas." This is not something you can check visually or fix yourself. A technician needs to connect a manifold gauge to the service ports and measure the suction and discharge pressures while the unit is running. If the pressures are below the recommended range for your specific refrigerant type, the system is undercharged.

Low gas is almost always caused by a leak somewhere in the system, not by the gas "running out" on its own. Refrigerant does not get consumed; it is sealed inside a closed loop. If the charge is low, refrigerant has escaped through a pinhole, a loose joint, or a damaged pipe. Simply topping up the gas without finding and fixing the leak means the problem will return, often within weeks. A thorough technician will add a small amount of dye or use an electronic leak detector to trace the source before recharging.

Compressor or capacitor failure

The compressor is the heart of the AC. If it is running but not compressing properly, or if the start capacitor has weakened, the unit will struggle to cool. Signs of a failing compressor include: the outdoor unit hums but the fan does not spin, the compressor trips on thermal overload after a few minutes, or the pipe temperatures feel wrong — both pipes are lukewarm when they should be one hot and one cold. Replacing a compressor is a major repair that can cost up to half the price of a new unit. In many cases, especially for older ACs, replacement rather than repair is the more economical choice.

A capacitor failure is cheaper to fix. The start capacitor gives the compressor the initial electrical kick to begin turning, and the run capacitor keeps it going. If either fails, the compressor may not start or may run at reduced efficiency. Capacitors cost a few hundred rupees and are quick to replace.

Frozen evaporator coil

An indoor coil that is frozen solid is a dramatic visual sign. Ice forms on the copper pipes and fins because the airflow is too low (clogged filter or a slow fan motor) or the refrigerant charge is low, causing the coil to get colder than normal. If you see ice on the indoor unit pipes, turn the AC off and let it thaw completely before investigating further. Running the unit on fan-only mode for a few hours speeds up the thaw. Once thawed, clean the filter and try again. If the ice returns, call a technician to check for a refrigerant leak or a failing fan motor.

When to call a professional

If you have cleaned the filter, checked the outdoor coil, and verified the settings, but the AC still does not cool properly, it is time to bring in a technician. A proper diagnostic visit includes checking pressures, electrical draw, temperature drop, and airflow. At Serve Tech, our HVAC service starts with a free inspection and a written quote, so you know what the problem is and what the fix costs before any work begins. We cover all major brands and carry common spare parts on the van so most faults are diagnosed and repaired in a single visit.