Air conditioner acting up? Start here.

The most frequent causes are a dirty filter (it blocks heat exchange), low refrigerant from a leak, a blocked outdoor unit with no room to reject heat, or a room larger than the unit's capacity. Start by cleaning the filter and clearing space around the outdoor unit. If it still won't cool, it's usually a refrigerant leak — that needs a technician with a leak detector, since simply topping up refrigerant without finding and sealing the leak means it comes back within months.

Check the breaker and outlet first — it's the most common cause and the cheapest to fix. Then check the remote's batteries and whether the unit's display shows a blinking error code. If the breaker trips every time you turn it on, stop trying — it can be a short in the board or compressor, and repeated attempts risk damaging the unit.

Almost always a clogged condensate drain, or one installed without the minimum slope — water that should drain out backs up and overflows the indoor unit. A dirty filter causes this indirectly too: the coil freezes, thaws at the wrong time, and the drain pan can't keep up. If the leak is at the outdoor unit, or persists with a clean drain, the refrigerant line insulation may be compromised.

A musty smell is biofilm — mold and bacteria — built up on the coil and drain pan, common when only the filter gets cleaned and the unit is never fully sanitized. A burning smell is more serious: it can be wiring, a capacitor, or the control board overheating. Shut it off at the breaker and call a technician — this isn't one to wait on.

Usually circuit overload — a breaker undersized for the compressor's startup current — a missing dedicated circuit, or an internal short in the unit. An air conditioner needs its own circuit sized for startup current, not an outlet shared with other appliances. If the circuit is already correctly sized and it still trips, the fault is in the unit, not the wiring.

Vibration is usually a loose outdoor-unit mount without rubber isolation pads, or a mounting surface that isn't rigid enough. Clicking on startup or shutdown is normal thermal expansion of the plastic housing. A continuous high-pitched whistle usually points to a refrigerant leak at a valve; a loud, low compressor noise can mean lost lubrication or end of service life.

A dirty filter forces the compressor to run longer for the same result — the easiest cause to rule out. Next, check whether the capacity (BTU) actually matches the room's area and sun exposure: an undersized unit never shuts off, and an oversized one short-cycles without stabilizing — both waste energy. An efficiency rating or inverter compressor helps, but doesn't make up for wrong sizing.

Before assuming it's broken, replace the batteries and check for anything blocking the line of sight to the unit's receiver — strong light hitting the receiver directly can interfere too. If another remote works fine and only this one fails, the unit's infrared receiver itself may be faulty.

Almost always low refrigerant from a leak, or restricted airflow — a dirty filter, a dirty coil, or a failing blower fan. Never run a frozen unit on cooling mode hoping it clears itself: turn it off, let it thaw naturally, clean the filter, and if it freezes again, get the refrigerant checked — running it frozen wears out the compressor.