Mini-split running but not cooling (or icing up)

Check these first

Trane, MRCOOL, Fujitsu, and Daikin all list the same few checks before a service call (Trane, MRCOOL, Fujitsu, Daikin):

The mode. The remote should say Cool. Fujitsu says that in Dry mode the fan runs slow and may stop now and then, so Dry can feel like no cooling on a hot afternoon. If you use dry mode on muggy days, switch back to Cool when it’s hot.
The setting. Set it a few degrees below the room. Fujitsu says Cool may not start if the setting isn’t lower than the room’s temperature.
The filters. With the power off at the breaker, slide them out and look. A clogged filter cuts the air going across the coil. The filters guide has the steps.
What’s in the way. Move curtains or furniture blocking the indoor unit. Outside, clear leaves and trim plants back. Trane says to keep plants two feet from the outdoor unit.
Quiet and economy settings. Fujitsu says the Quiet fan setting lowers cooling, and its Economy mode tops out at about 70% of normal output.
Other indoor units. On a system with several indoor units, one can’t heat while another cools. Fujitsu says the unit started second stops and its light flashes. Don’t run one in Heat while another is in Cool or Dry.
The outdoor unit. Give it a few minutes after any restart. Fujitsu says the compressor waits about 3 minutes after the power comes back, to protect itself. Then, while the indoor unit is calling for cooling, check that the outdoor unit is running and blowing air. If it isn’t, check the breaker. A Carrier dealer quoted on Carrier’s repair page says an outdoor unit that won’t run usually needs a professional (Carrier). If the breaker trips again, leave it off. See AC breaker keeps tripping.

A room that got hot all day also takes a while to come down.

Measure the air

You can put a number on “not cooling” with a cheap thermometer. MRCOOL suggests measuring the air going into the indoor unit and the air coming out of the louvers. It says a healthy system usually shows a difference of 8 to 30°F, and a smaller difference can mean an airflow problem, low refrigerant, or a failing part (MRCOOL). Write the numbers down for whoever comes to look. Delta T explains what that difference means.

If you see ice

Frost or ice on the coil behind the louvers, or on the copper lines, means the coil got below freezing. Carrier says a frozen system won’t fix itself, and running it strains the compressor (Carrier).

Turn cooling off. If water is dripping, turn it off at the breaker.
Let the ice melt on its own. Don’t chip it, heat it, or pour hot water on it. Carrier says light frost can take 1 to 3 hours to thaw and heavy ice 4 to 8 hours or more.
Put towels or a pan under the unit. Melting ice makes a lot of water. See leaking water.
While it thaws, wash the filters with the power off.
Don’t restart cooling while any ice remains. Even after it has fully thawed and the clean, dry filters are back in, restart it only if a technician or your manual says it’s OK. Otherwise, leave it off and call for service. If it is restarted, watch it closely. Turn cooling off again and call if ice returns, airflow falls, water leaks, or it doesn’t cool normally. Don’t keep restarting it.

Why a mini-split ices up

The coil freezes when too little warm air reaches it, or when the refrigerant inside it runs too cold.

Too little air. Dirty filters are the first suspect. Gree says a dirty filter can freeze the coil (Gree), and its E2 code means freeze protection: check the filters (Gree). But the filters aren’t the only thing in the air’s path. Trane says the coil itself can get covered with dust and dirt and freeze up (Trane). Gree’s guide for contractors says buildup on the blower wheel reduces airflow and cooling (Gree). Washing the filters doesn’t reach either one.

Low refrigerant. Refrigerant doesn’t get used up. Carrier says if it’s low, there’s a leak (Carrier). Gree lists a refrigerant leak as a cause of freezing up (Gree), and MRCOOL lists ice on the copper lines as a sign of low refrigerant (MRCOOL).

The flashlight check helps you guess which. With the power off, shine a light up through the louvers at the blower wheel, as the cleaning guide shows. A thick coat on the blades means the cleaning comes first. The light can’t show you the coil well, though. Clean filters and a clean-looking wheel with ice that keeps coming back still leaves a dirty coil, low refrigerant, or a failing part, and a technician has to sort out which.

Refrigerant is a technician’s job

EPA requires Section 608 certification to recharge a system, and refrigerant for home systems is sold only to certified technicians. Hooking up hoses or gauges counts as that work too. EPA also says to ask the technician to find and fix the leak instead of “topping off” the system (EPA). Recharge cans and leak sealants both connect to the lines, so they’re out too.

The water dripping from a mini-split is never refrigerant. It’s water pulled from the air, and a refrigerant product won’t stop a drip. The one link is ice: Gree says low refrigerant can freeze the coil, and it drips as it thaws. The leaking water guide covers the drips.

Cold air, but the room falls behind on hot afternoons

If the air coming out is cold and nothing is iced, but the room creeps up on the hottest afternoons, the unit may just be working near its limit. Fujitsu lists sun through a window, other heat sources, many people, and open doors as reasons for weak cooling. Why an AC can’t hold the setting on the hottest afternoon explains what’s normal on a peak afternoon. It’s written for central air, but watch for the same things: a sudden change, air that turns warm, or a room that won’t recover after sunset.

When to call

The coil iced up, and neither a technician nor your manual has said it’s OK to restart. Call too if ice comes back after a restart they approved.
The outdoor unit won’t run while the indoor unit calls for cooling.
The air coming out is barely cooler than the room.
The unit shows a code or a blinking light that doesn’t clear. See error codes.
The breaker trips again, or you smell burning. Turn it off at the breaker and leave it off.

For a central air system, see why an AC runs but doesn’t cool and what to do if your AC is frozen.