Quick answer
When one mini-split head is not working and the others are, check the modes first. Every head on a multi-zone system must be in heat, or every head in cool; the first head switched on sets the mode and a conflicting head sits in standby. If the modes match, the cause is that head's expansion valve, crossed wiring or piping from installation, or a failed sensor, fan motor or board.
One mini-split head not working while the rest of the system runs is a mode conflict until proven otherwise. A multi-zone outdoor unit has a single compressor and a single reversing valve. It can heat or it can cool, but it cannot do both at once, so any head asking for the opposite of what the system is doing gets parked.
Mode conflict: every head must heat, or every head must cool
On most brands the first head turned on sets the system's mode. A head that then asks for the opposite goes into standby: its light blinks, the fan stops or idles, and it will not respond until the modes agree. Some systems can be set at installation to give priority to one head, or to heating, instead.
The usual trigger is Auto. In October a sunny south room drifts warm, its head decides to cool, and the head in the north bedroom that wants heat is locked out. Set every head to Heat for the winter and Cool for the summer, and leave Auto alone. Dry mode counts as cooling, and a room that needs nothing should have its head switched off at the remote.
Check this yourself first
- Read every remote: all heads in the same mode, none in Auto.
- Restart in order: turn everything off, wait five minutes, then start the problem head first, by itself, in the mode you want.
- Filter: slide it out and rinse it. A matted filter makes one head weak while the others are fine.
- Remote: fit fresh batteries and confirm the head beeps when you press a button.
- Display: note any blinking light or code before you reset anything. What a blinking mini-split light means helps you read it.
- The one-at-a-time test: run each head alone for 15 minutes and note which head on the wall actually gets hot or cold.
If no head heats at all, the fault is in the shared outdoor unit; that is covered in why a mini-split is not heating.
Crossed wiring and line sets from the installation
If the one-at-a-time test shows head A calling and head B getting warm, the installation is crossed. Each head has its own pair of copper lines and its own communication cable back to a lettered port on the outdoor unit or branch box. When a cable lands on port A and its line set on port B, the outdoor unit sends refrigerant to the wrong room.
The giveaway is history: the head has never worked properly, or the fault only shows when one head runs alone, because with every head running each one gets refrigerant anyway. The repair is usually moving a cable to the matching terminal, with no refrigerant work.
Stuck expansion valves and branch-box ports
A multi-zone system meters refrigerant to each head through its own electronic expansion valve, one per port, inside the outdoor unit or in a branch box on larger systems. Each valve is a needle driven by a small stepper coil. Stuck closed, that head gets no refrigerant and blows room-temperature air while its neighbours work. Stuck open, the head heats or cools when it is switched off, and the room overheats.
The coil clips onto the outside of the valve and can be swapped without opening the refrigerant circuit. A valve body that is jammed has to be cut out and a new one brazed in, which means recovering the charge, pressure-testing and recharging by weight.
Faults inside the head, and the minimum-capacity limit
- Thermistor: each head reads room and coil temperature with small sensors. One that drifts out of range makes the head stop early, refuse to start or show a code.
- Indoor fan motor: the coil gets hot or cold but little air moves. You may hear the motor hum or the wheel scrape.
- Indoor board or receiver: the head is dead or ignores the remote while the rest of the system carries on.
- A refrigerant leak at that head's flare joints: a multi-zone system shares one charge, so the leak eventually weakens every head, but it often shows first at one.
The last cause is a design limit, not a breakage. A multi-zone condenser can only turn down so far, and most require at least two heads connected plus a minimum total indoor capacity listed in the installation manual. When one small head calls by itself on a large outdoor unit, the system makes more heat than that head can use, so it cycles on and off and the room swings. The single-zone versus multi-zone comparison explains why separate single-zone systems avoid this.
What it costs to fix one dead mini-split head
A mode conflict costs nothing. For the rest, the diagnostic visit is $90–$150 and is credited to the repair, and most single-head repairs, such as a sensor, a fan motor at $300–$700 or a board at $400–$800, fall inside the $150–$800 band. Expansion-valve bodies, branch-box parts and any job that needs the refrigerant recovered are quoted in writing first.
It is safe to keep running the other heads while you wait, unless the system shows a leak: ice on a line, oily residue at a joint, or every head growing weaker. In that case shut it down.
ZK Mechanical works on multi-zone Mitsubishi, Fujitsu and Daikin systems, including ones another installer put in. The ductless mini-split page describes the service, mini-split repair covers the diagnostic visit, and you can tell us which head and what it is doing or call (647) 801-1252.