Mini-split not cooling or heating? Here is how to find the cause

Why is my mini-split running but not cooling?

Start with the filters, because a dirty one is the single most common reason a mini-split blows weak or warm air, and it is free to fix. Unlike a central system, a mini-split does not use a disposable pleated filter. Behind the front flap on the indoor head are washable mesh screens, and they clog fast, in some homes within two weeks of heavy use. Lift the flap, slide the filters out gently so you do not shake dust everywhere, rinse them under warm water or vacuum them on low, let them dry fully, and slot them back in. A clogged filter chokes airflow enough to cut cooling, freeze the coil, and grow the musty smell people often notice at the same time.

If clean filters do not fix it, work through the other free checks. Confirm the remote is set to Cool, not Fan or Dry, and that the setpoint is several degrees below the room temperature, because in Fan or Dry mode the unit moves air without actively cooling. Replace the remote batteries. Then walk outside and look at the outdoor unit: leaves, grass clippings, or a stack of patio furniture against it block the airflow it needs to dump heat. Clear a couple of feet around it. These checks, the filters plus the settings plus the outdoor unit, resolve the large majority of weak-cooling cases without a service call.

What is left after that usually needs a technician. The indoor coil and blower wheel collect grime and mold over time and cut output, and they sit deep enough inside the head that cleaning them means partly taking the unit apart, which is a pro job. A refrigerant leak is the other big one, and on a mini-split it most often shows up at the flared connections where the line set joins each unit, a known weak point on installs that were not torqued right. Low refrigerant makes the unit run without cooling, can ice up the line into a solid block, and is never something to just top off, the leak has to be found and sealed first.

Why did my mini-split stop heating in cold weather?

Before you treat this as a failure, rule out the most common false alarm: defrost mode. When it is cold and damp, frost builds on the outdoor coil, and the system periodically reverses itself to melt it off. During those five to fifteen minutes the indoor unit stops blowing warm air, or blows cool, and you will often see what looks like smoke or steam rising off the outdoor unit. That is water vapor, not a problem. The cycle runs as often as every thirty to ninety minutes in the right weather, and warm air comes back on its own each time. A short pause with steam is the system working correctly, not breaking down.

Real heating problems have the same free causes as cooling, so check them in the same order: wash the filters, confirm the remote is on Heat with the setpoint above room temperature, replace the batteries, and clear snow or debris off the outdoor unit. Two things are specific to heating. First, a standard mini-split loses heating power as the outdoor temperature drops and may not keep up in deep cold unless it is a cold-climate model built for it. Second, most ductless mini-splits have no backup heat strip the way a central heat pump does, so when one runs short in a cold snap there is nothing to fall back on unless you have a separate heat source. If your heat pump is a central ducted system rather than a ductless one, our guide on a heat pump that will not heat covers that side.

The sign that something is actually wrong is a thick block of ice on the outdoor unit that never clears. Light frost that comes and goes is normal; a solid sheet of ice that the defrost cycle cannot melt points to low refrigerant, a drainage problem, or a stuck defrost control, all of which need a tech. If the unit is set to Heat but only ever blows cold, the reversing valve that switches the system between heating and cooling may be stuck, which is also a repair, not a setting.

My mini-split is completely dead. What do I check?

When the indoor unit will not respond at all, start with the remote, because dead batteries are the most common cause by a wide margin. Then use the trick most owners do not know about: there is a small manual or emergency ON button hidden behind the front flap of the indoor unit, usually labeled something like Auto or Test. Press it. If the unit kicks on from that button but ignores the remote, the unit itself is fine and your problem is the remote or its receiver, not the system.

If the manual button does nothing either, the unit has lost power. Find the breaker and, if it has tripped, push it fully off and then back on once. If it trips again right away, stop and call a tech, because a repeat trip means a real electrical fault, not a fluke. Check the outdoor disconnect switch, a small box within a few feet of the outdoor unit, and make sure it is on. If the outdoor unit is fed from an outdoor outlet, a rainstorm can trip its GFCI, so look for a popped reset button. If power is confirmed everywhere and the unit is still dead, the control board has likely failed, which is a tech repair.

What do the blinking lights on my mini-split mean?

A mini-split tells you what is wrong by flashing the lights on the indoor unit or showing a code on the remote or a wall display. The flashes are not random. Count how many times a light blinks before it pauses and repeats, and note whether it is the operation light, the timer light, or both. That pattern maps to a specific fault code, and every brand has its own list, so the number means different things on a Daikin than on a Mitsubishi or an LG.

You do not need to decode it yourself, but you should write it down. The codes generally fall into a few buckets: a communication error between the indoor and outdoor units, a sensor fault, or a high or low refrigerant pressure problem. A slow, steady blink once every couple of seconds is usually just standby and not a fault at all. The useful move is to record the exact pattern or code from the manual, then hand it to the technician, because it points them straight at the problem and shortens the diagnosis. A few codes are homeowner-clearable, a freeze-protection code that clears after you clean the filter and let the ice melt, for instance, but most sensor and board codes need a tech.

Why is my mini-split leaking water inside?

Water dripping from the indoor head onto the wall or floor almost always means the condensate drain line is clogged. The unit pulls humidity out of the air, that water is supposed to run downhill and outside through a small drain hose, and when algae and dust block the line, it backs up and overflows. You can often clear it by running a wet/dry vacuum on the outdoor end of the drain line to pull the clog out, and flushing the line every few months keeps the algae from coming back.

The other common cause is a bad install. The indoor unit has to hang dead level and the drain hose has to slope downhill the whole way, with no kinks. If the head was mounted tilted or the hose sags, water escapes instead of draining, and fixing that means re-mounting the unit, which is a pro job. One more: if a dirty filter froze the coil into ice, it will pour water when it thaws and overwhelm the drain pan, so if you see ice, shut the unit off, let it thaw, and clean the filter before running it again.

Why does my mini-split smell musty?

A musty, dirty-sock smell is mold and mildew growing on the damp indoor coil and blower wheel. A mini-split coil stays wet after cooling, and the dust and pet hair that gets past a dirty filter gives mold something to grow on. The first fix is to clean the filters and then run the unit in Fan-only mode for a few hours after cooling, which dries the coil before mold takes hold. Many units also have a Dry mode that helps on humid days.

If the smell sticks around after the filters are clean, the blower wheel itself needs a professional deep clean, because that part sits too far inside the head to reach without taking it apart. One safety note: a musty smell is mold, but a burning, chemical, or electrical smell is not, and that one means shut the unit off and call, because it points to a component overheating rather than mold.

What do mini-split repairs cost, and when should I call a pro?

Once you are past the filters, the settings, the outdoor unit, and the drain line, a mini-split problem is a technician's job. The line is simple: if the fix means opening the unit or touching refrigerant, it is not a do-it-yourself repair. That covers any leak or recharge, an iced-up system from low charge, an error code that will not clear, a blower-wheel deep clean, and anything electrical like a capacitor, control board, reversing valve, or compressor. These are installed prices, parts and labor together.

Repair	Installed cost	What it fixes
Condensate drain clearing	$150 to $350	The clogged drain line behind most indoor water leaks.
Capacitor (outdoor unit)	$105 to $300	The starting jolt for the outdoor fan and compressor.
Indoor deep clean (coil and blower wheel)	$150 to $450 per head	Removes the mold causing a musty smell and restores airflow.
Refrigerant leak repair and recharge	$250 to $600+	Seals a leaky flare joint and refills the charge. More if lines must be replaced.
Control board	$300 to $800	The electronics that run the unit. Mini-split boards are model-specific and pricey.
Reversing valve	$550 to $1,400	The valve that switches between heat and cool. Expensive because it is a sealed-system job.
Compressor	$800 to $1,550	The heart of the outdoor unit. On an older head, often near the cost of replacing it.

The best money you spend on a mini-split is the time it takes to wash the filters every couple of weeks and keep the outdoor unit clear, because that prevents most of the failures above. When a big-ticket repair like a compressor or reversing valve lands on a mini-split that is already past ten years old, the bill can approach the cost of a new head, so it is worth pricing a replacement before you commit. Our replace-versus-repair calculator helps you run that comparison.