Quick answer
A mini-split that is not cooling is most often short of airflow or short of refrigerant. Wash the mesh filters, confirm the remote is on COOL and not DRY, FAN or AUTO, and clear the outdoor coil. If the air leaving the head is less than about 8 °C cooler than the room, or ice forms on the small copper line, a technician needs to find a leak.
Working out why your mini-split is not cooling takes one measurement and three checks. Hold a kitchen thermometer in the air leaving the head, then in the air at the intake on top. A healthy unit on COOL delivers air at least 8 °C colder than it takes in. A smaller gap with weak airflow points to dirt; a smaller gap with strong airflow points to the refrigerant side or the outdoor unit.
Check the remote mode, the filters and the outdoor coil first
- Mode: the remote should show COOL, the snowflake. DRY runs the fan slowly to remove moisture and barely drops the temperature, FAN moves air with no cooling, and AUTO can sit idle or even heat when the room is near the setpoint.
- Setpoint: set it 3 °C below the room temperature while you test, with the fan on high.
- Filters: lift the front panel, slide out the washable mesh filters, rinse them in the sink and let them dry. With pets or a kitchen nearby, that is a job for every two to four weeks in summer.
- Outdoor coil: look through the grille at the back and side of the outdoor unit. Cottonwood fluff, grass clippings and dryer lint mat the fins; rinse them gently with a garden hose, never a pressure washer.
- Clearance: move bins, planters and stored furniture so the unit has open air around it and is not breathing its own hot exhaust.
- Load: one head sized for a bedroom will not pull down an open-plan main floor on a 32 °C afternoon with the doors open.
Dirt you cannot reach: a matted indoor coil and blower wheel
Clean filters do not mean a clean unit. Behind them, the indoor coil's fins and the long barrel-shaped blower wheel collect a felt of dust and biofilm that washing the filters never touches. Each blade of that wheel is a shallow scoop, and once the scoops fill in, airflow drops sharply even though the motor spins at full speed. The signs are a head that sounds normal but barely moves air, black specks on the louvre, and the musty odour explained in why a mini-split smells bad.
The cure is a deep clean, with the head bagged and the coil and wheel washed in place, which restores airflow in one visit. If the unit is more than three or four summers old and has never had one, this is the likeliest reason it cools worse than it used to.
Low refrigerant: leaking flare connections and ice on the small line
A mini-split does not use up refrigerant, so a low charge means a leak, and on these systems the leak is usually at a flare connection. The copper lines join the indoor and outdoor units with flared ends clamped under brass nuts. A flare that was cut rough, or tightened by feel instead of with a torque wrench, seeps for months or years before the charge drops far enough to notice. It is the most common installation defect on ductless equipment.
The tell is frost or ice on the smaller of the two copper lines where it meets the outdoor unit, sometimes with an oily film on the nut. Turn the system off when you see it, because running low on charge overheats the compressor. The repair is to find the leak, remake the flare, pressure-test with nitrogen, pull a vacuum and weigh in the charge listed on the nameplate. Topping up without fixing the joint only resets the clock, and refrigerant handling in Ontario requires an ODP card.
Outdoor fan, inverter board and multi-zone expansion valve faults
When the indoor head blows room-temperature air and the outdoor unit is silent, or starting and stopping every minute, the fault is outside. A seized outdoor fan motor lets the coil overheat until the unit trips on pressure protection. A failed inverter board, the electronics that drive the variable-speed compressor, usually leaves a flashing lamp or a code on the head; what the blinking light means explains how to read it.
On a multi-zone system, each head is fed through its own electronic expansion valve inside the outdoor unit or a branch box. A valve stuck closed starves one head, which blows warm while the others cool normally. A valve stuck open does the reverse: a head that is switched off stays cold, sweats and hisses. Either way the charge is fine and the fix is the valve or its coil, not more refrigerant.
When to call, and what a mini-split cooling repair costs
A mini-split that cools weakly but shows no ice is safe to run at a higher setpoint until a booked visit. Shut it off and call sooner in any of these cases.
- Call now for ice on the indoor coil, the copper lines or the outdoor service valves.
- Call now for water dripping from the head along with poor cooling.
- Call now if the outdoor unit trips its breaker or makes a grinding or buzzing noise.
- Call now for an error code that returns after one five-minute power reset.
- Diagnostic visit: $90–$150, applied to the repair.
- Flare leak repair with pressure test and recharge: $300–$800 and up, depending on where the leak is and how much refrigerant was lost.
- Outdoor fan motor replacement: $300–$700.
- Most other cooling faults: between $150 and $800, with inverter boards and expansion valves quoted in writing because the parts are model-specific.
ZK Mechanical repairs and deep-cleans ductless systems from Burlington to Hamilton and Mississauga, with the price confirmed before work starts. The ductless mini-split page covers repair, cleaning and replacement, the heat pump answers hub has the related questions, and you can book a visit online or call (647) 801-1252.