A window air conditioner that runs but does not cool the room is very often struggling against restricted airflow or a poor seal, not a refrigerant fault. Unlike central systems, window units are also frequently let down by simple installation issues — the seal around the unit, or its tilt — that are easy to overlook and easy to fix.
Common causes
| Cause | How likely | What to look for |
|---|---|---|
| Dirty air filter | Very common | Weak airflow from the vents; filter visibly clogged with dust |
| Dirty coils | Common | Reduced cooling that has developed gradually |
| Poor seal around the unit letting outside air in | Common | Gaps around the sides or top of the unit visible from inside or out |
| Unit not tilted correctly for drainage | Common | Should tilt slightly toward the outside; water can back up or drainage can be affected otherwise |
| Thermostat/temperature setting misjudged | Occasional | Set to a fan-only mode, or a target temperature above room temperature |
| Undersized unit for the room | Occasional | Room is larger than the unit’s rated coverage |
| Low refrigerant from a leak | Less common | Ice forming on the coil, cooling capacity gradually worsening over a longer period |
How to fix it
- Clean or replace the filter
Most window units have an easily removable filter behind the front grille. Wash a reusable one or replace a disposable one, and do this monthly during heavy use.
- Check the mode and temperature setting
Confirm it is set to COOL rather than FAN ONLY, and the target temperature is genuinely below the room’s current temperature.
- Inspect the seal around the unit
Look for gaps at the sides, top, or bottom where warm outdoor air can leak in around the unit. Foam weatherstripping seals these cheaply and effectively.
- Check the tilt
Most manufacturers recommend a slight backward tilt so condensation drains outside rather than pooling inside. Check this against your model’s manual.
- Clean the coils if accessible
With the unit unplugged and, ideally, removed from the window per the manual’s instructions, gently clean visible dust from the coils with a soft brush or a coil cleaner spray.
Installation issues specific to window units
Because window units sit half-in and half-out of the house, installation quality affects performance in ways that do not apply to central or split systems. A gap of even a centimetre around the frame lets warm outdoor air flow straight back into the room the unit is trying to cool — this alone can make a perfectly working unit feel weak.
When to call a technician
If the filter and coils are clean, the seal is tight, and the unit is tilted correctly but it still will not cool — or if you see ice forming on the coil with everything else checked — have a technician assess the refrigerant charge and compressor.
Stop and call a qualified appliance technician or electrician if you find scorched or melted wiring, smell burning, see water that has reached electrical components, or if the fault returns immediately after a repair. Never bypass a safety device — thermal fuses, door interlocks and flood switches fail for a reason, and defeating one converts a broken appliance into a fire or flood risk.
