Window AC not cooling? Here is how to fix it

Why is my window AC running but not cooling?

Start with the filter, because a dirty one is the most common cause by a wide margin and it is free to fix. A window unit has a washable foam or mesh filter right behind the front grille, and it clogs fast, within a couple of weeks in heavy use. Pop off the front panel, slide the filter out, rinse it under the tap, let it dry completely, and put it back. A clogged filter starves the airflow the unit needs to cool, and a badly clogged one can even freeze the coil into ice. Make this your first move every time.

Check the simple things next. Confirm the unit is set to Cool, not Fan, because Fan mode just blows room-temperature air, and set the temperature below the current room temperature so the compressor actually runs. One setting fools people: in eco or energy-saver mode, the compressor and fan shut off completely once the room hits the setpoint and only kick back on when it drifts up, so the unit blowing nothing for stretches is normal in that mode, not a fault. Then look at the coils. The front coil inside and the rear coil facing outside both collect dust, pollen, and leaves, and a clogged coil cannot move heat. The rear coil is the one that faces outdoors and gets the worst of it. With the unit unplugged, brush or vacuum the coils gently so you do not bend the thin fins, and for a heavier buildup, spray on a coil cleaner, let it sit a few minutes, and rinse it off carefully without getting water near the controls. If the coil has iced over, that is its own problem, covered just below.

Two installation issues quietly hurt cooling. If the unit is undersized for the room, or you are trying to cool an open floor plan with a unit meant for one bedroom, it will run constantly and never catch up on a hot day. And if there are gaps around the unit in the window, hot outside air leaks straight in and fights everything the AC is doing. The accordion side panels that came with the unit are the usual weak spot, because they are thin and seal poorly, so press them out to fill the opening and add weatherstrip foam or caulk anywhere you can feel air or see daylight around the cabinet. A unit cooling against a steady leak of hot air will always seem weak no matter how clean it is. If you are not sure your unit is the right size for the space, our BTU sizer shows the size a room actually needs.

Why is my window AC leaking water inside?

Nine times out of ten this is the tilt. A window unit is supposed to sit level side to side but tilt just slightly downward toward the outside, roughly a quarter inch, so the condensate it pulls from the air drains out the back instead of pooling and dripping into your room. If the unit is dead level or tilted back into the house, the water has nowhere to go but inside. Re-seat the unit with a slight downward slope to the outside and the indoor dripping usually stops.

The other cause is a clogged drain. There is a small drain channel and weep hole in the base pan that can choke with dirt and algae, backing the water up until it spills over the indoor edge. Clear it with a pipe cleaner or a long cotton swab. One thing that is not a problem: water dripping outside the back of the unit is normal. Many units are designed to fling that condensate onto the rear coil to help with cooling, so a little water out the back is by design, not a leak.

Why is my window AC icing up?

Ice on the indoor coil always traces back to one of three things: not enough air moving across the coil, the fan set too low, or running the unit when it is too cool outside. When airflow drops, the coil gets cold enough to freeze the moisture in the air right onto itself, and once a little ice forms it snowballs, blocking more airflow and making more ice. A dirty filter is the usual starting point, so a frozen unit and a clogged filter often go together.

The fix is to stop cooling and let it thaw. Switch the unit to fan only, or just turn it off, and give the ice time to melt fully, which can take anywhere from an hour to most of a day depending on how much built up. Do not keep running it on Cool while it is frozen, because that strains the compressor and can damage the fan. Once it is clear, clean the filter, run the fan on high rather than low, and do not run the unit for cooling when it is below about sixty degrees outside, which is the other common reason a coil ices over.

Why won't my window AC turn on at all?

Before you assume the unit is dead, check the plug. Most window-AC power cords have a chunky plug with Test and Reset buttons built into it, a safety device that cuts power if it senses a fault in the cord. When it trips, the unit goes completely dead, and pressing Reset brings it right back. This is the single most useful thing to know about a window unit that will not turn on, because it looks like a dead appliance and is actually a tripped safety. If the reset will not hold, or the plug looks scorched or feels hot, stop using it, because the cord itself has failed.

If the plug is fine, check that the outlet is live and the breaker has not tripped. Window units pull a lot of power, especially in the instant the compressor starts, when the surge can briefly draw several times the running amps. Run one on a circuit shared with a microwave, a hair dryer, or a space heater and that startup surge trips the breaker. The unit then looks dead when it simply lost power. If the breaker keeps tripping, move other appliances off that circuit, or give the AC its own. A larger unit, in the ten-to-twelve-amp range, really wants a dedicated circuit so it is not fighting anything else.

Plug it straight into the wall, never into a thin extension cord. This is not a nicety, it is a genuine fire hazard with an appliance that draws this much current, and fire departments trace house fires to it every summer. A too-thin cord overheats under the sustained load and can ignite. If you truly must use a cord, it has to be a short, heavy-gauge one rated above the unit's draw, plugged directly to the wall at the other end.

Why is my window AC noisy or smelly?

Match the noise to the cause. A rattle or vibration is usually loose panel screws or a unit that is not seated firmly and level in the window, both of which you can fix by tightening and re-seating it. A buzzing or humming is electrical, a loose connection or a struggling part, and a grinding or screeching is a failing fan motor bearing, which on a cheap unit is usually a reason to replace rather than repair. Always unplug the unit before you tighten or re-seat anything.

A musty, dirty-sock smell is mold growing on the damp coil and filter, which is common after the unit has sat unused. Clean the filter and the coil, and run the unit on fan only for a while after cooling to dry the coil out. A neglected unit can blow mold spores into the room, so this is worth doing for your air as much as the smell. One smell is different: a burning or electrical smell means you unplug the unit immediately and do not run it again, because that points to wiring or a motor overheating, not mold.

Should I repair my window AC or just replace it?

This is where a window unit is different from central air, and the math is usually simple. A new window AC costs a few hundred dollars, while a technician's diagnostic visit alone runs most of the way to that before any parts. So the moment a repair involves a real part, the numbers tip toward replacing the unit. Cleaning, re-tilting, resetting the plug, tightening panels, and swapping a filter are all worth doing because they are free or nearly so. A failed compressor, a control board, a fan motor, or a refrigerant leak are not, because each one costs more than the unit is worth.

Refrigerant deserves a special note. A window unit is sealed at the factory and is not meant to be recharged, and by law only a certified technician can handle refrigerant at all. A unit that is low on refrigerant has a leak, and on a sealed window unit that is effectively the end of the road, not a repair worth paying for. Window units last about eight to ten years, so once yours is in that range and needs anything beyond a cleaning, a new and more efficient unit is almost always the better buy. If you are weighing the same question on a central system instead, our replace-versus-repair calculator runs that comparison.

How do I keep a window AC cooling well?

A little routine prevents most of these problems. Clean the washable filter every couple of weeks during the season, and clean the coils once a season with a soft brush and a gentle rinse. Make sure the unit is sealed in the window with no air gaps and tilted slightly out so it drains. Do not run it when it is below about sixty degrees outside, because the coil will ice up and the cold can damage the compressor. And give it a shaded window if you can, since a unit baking in direct afternoon sun has to fight its own heat load.

Keeping it clean is also an air-quality habit, not just a performance one. A neglected unit grows mold on the damp coil, the filter, and the drain pan, and then blows those spores into the room every time it runs, which can set off sneezing, congestion, and asthma. The same filter and coil cleaning that keeps it cooling well is what keeps it from becoming a mold machine, so it is worth staying on top of.

One safety habit matters more than the rest: a window unit hangs mostly outside the window and is heavy, so on any upper floor it needs a proper support bracket so it cannot fall. At the end of the season, either pull the unit and store it clean and dry indoors, or cover just the outside portion with a breathable cover, never sealed plastic, which traps moisture and breeds rust and mold.