None of these can be confirmed without looking at the system - they're the possibilities a technician works through.

• A dirty filter. The most common and most easily addressed. A clogged filter restricts how much air can pass through.
• Restricted airflow elsewhere - blocked vents, furniture against a grille, or a fouled coil limiting how much air moves.
• A dirty cooling coil. Dust settling on the coil reduces how effectively it transfers heat, so the air leaving the unit isn't as cold.
• Thermostat or settings. Worth ruling out early - a wrong mode or set temperature is a genuinely common cause.
• Drainage problems. A restricted drain can cause the system to behave abnormally, and often shows up as water before it shows up as poor cooling.
• A refrigerant problem. If the refrigerant charge is low, the system runs but cools poorly. This needs a technician.
• A fan or blower problem. If the blower isn't moving air properly, cooled air isn't reaching the room.
• An electrical or control fault. A failed component or control board can leave part of the system running while another part isn't.
• An outdoor unit issue. On split and central systems, if the outdoor unit isn't running or is heavily blocked, the system can't reject heat and cooling drops off.

These are safe to check, and they occasionally solve it outright.

• The thermostat - that it's in cooling mode and set well below the current room temperature.
• Whether the system is actually running at all, or whether it's cycling on and off unusually.
• Obvious airflow obstructions - furniture, curtains or boxes blocking a vent or return grille.
• The condition of the filter, if it's in an accessible position you can reach safely.
• Whether the problem affects one room or the whole home, which is useful information to give a technician.

Beyond that, leave it. Don't open electrical panels, don't attempt anything involving refrigerant, and don't dismantle the unit or bypass any safety device. If you're unsure, that's the point to book a visit.

If cooling has dropped off gradually rather than suddenly, airflow feels weak, and the system hasn't been serviced in a while, cleaning is a reasonable first step. Dust build-up on filters and coils genuinely does reduce cooling performance, and it accumulates quickly in a system that runs most of the year.

If you're weighing up which service that means, our basic vs deep cleaning guide explains the difference, and our AC cleaning options list what each one covers.

• Cooling stopped suddenly rather than fading over weeks.
• The system has recently been cleaned and it still isn't cooling.
• There are unusual noises, or the unit trips, cuts out or won't start.
• Ice is forming anywhere on the system.
• The outdoor unit isn't running.

An AC that isn't cooling is not automatically just dirty. Repeatedly cleaning a system with an underlying fault costs money and doesn't fix it. A diagnostic visit establishes what's actually wrong, and the repair is quoted in writing before any work starts - you decide what happens next. See how AC repair works for what to expect.