Short answer: If the fan runs and you feel airflow but it's not cold, the top causes are a dirty air filter restricting airflow, a frozen or dirty evaporator coil, or a unit that's undersized for the room and running constantly without ever catching up.
1. Dirty air filter
A clogged filter restricts airflow across the cold coil, reducing how much cool air actually reaches the room — this is the most common and easiest cause to fix.
2. Frozen or dirty evaporator coil
Reduced airflow (often from that same dirty filter) can cause the coil to ice over, which then blocks airflow even further in a worsening cycle.
3. Undersized unit for the room
A unit with too few BTU for the room's size will run constantly and still struggle to reach the set temperature, especially in direct sun or a room with poor insulation.
4. Blocked or dirty outdoor-facing coil
Dust, debris, or nearby plants blocking the exterior-facing coil reduce the unit's ability to reject heat outside, directly reducing cooling performance.
Clean or replace the filter
Remove the filter and wash it (if reusable) or replace it (if disposable), then let it dry fully before reinstalling.
Thaw a frozen coil if present
Turn the unit off (fan-only mode if available, or fully off) and let any ice on the coil melt fully — this can take a few hours — before resuming normal cooling operation.
Clear both the room-facing and outdoor-facing airflow paths
Make sure nothing blocks air intake or exhaust on either side of the unit, indoors or outdoors.
Reassess sizing if problems persist
If the unit runs constantly without reaching temperature even with a clean filter and clear coil, use a BTU calculator to check whether it's actually sized appropriately for the room.
Safety warning
If you suspect a refrigerant leak, do not attempt to open the sealed system yourself — this requires EPA-certified handling. Always unplug the unit before removing panels beyond the filter access point.Every 2–4 weeks during regular use is a good baseline, more often in dusty environments or homes with pets.
Yes — an undersized unit will run constantly trying to reach the set temperature, using more energy and wearing out faster, even though it's technically 'working' the whole time.
Occasional light frost in very humid conditions can happen, but regular or heavy ice buildup points to restricted airflow (usually a dirty filter) that should be addressed rather than left to recur.
Home Solutions Editorial Team
Reviewed against manufacturer documentation. Updated August 6, 2026. How we write guides
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