Three indoor units running perfectly. The fourth — nothing. It’s blowing air, but it’s room temperature at best. Maybe the display is flashing an error code, maybe it’s completely blank. Either way, one unit on the multi-split system has dropped out and the rest carry on like nothing happened.
This is one of the most common call-outs we get on multi-split and VRV/VRF systems in London offices. The good news: because the other units still work, the outdoor unit is almost certainly fine. The problem is isolated to that one indoor unit or its connection back to the outdoor board. That narrows things down considerably.
Communication Fault Between Indoor and Outdoor PCB
This is the most common cause by a wide margin. Multi-split systems rely on serial communication between each indoor unit and the outdoor controller. If that communication link drops — due to a loose wire, corroded terminal, or a failed PCB — the outdoor unit simply stops sending refrigerant to that indoor unit.
On Daikin, Mitsubishi, and most other VRV/VRF systems, the outdoor unit keeps an error log that identifies exactly which indoor unit has faulted. That’s always the first thing we check. The error code will usually tell us whether it’s a communication error (often an E or U code on Daikin, or an A-series code on Mitsubishi) or something else entirely.
The fix might be as simple as re-terminating a loose connection on the indoor unit’s PCB. Or the PCB itself might need replacing. Either way, the diagnostic is straightforward once you’re reading the error codes properly.
Failed or Stuck Electronic Expansion Valve
Each indoor unit on a multi-split system has its own electronic expansion valve (EEV) that controls how much refrigerant flows through its heat exchanger. If that valve fails — stuck closed, stuck open, or responding erratically — the unit won’t cool properly even though everything else appears normal.
A stuck-closed valve means zero refrigerant flow. The unit blows air but there’s no cooling at all. A stuck-open valve can cause the coil to flood with liquid refrigerant, which may trigger a protection shutdown or produce wildly inconsistent temperatures. Either scenario means the valve or its coil driver needs replacing.
Wiring Termination Fault on That Specific Unit
Multi-split installations can run tens of metres of communication and power cable from the outdoor unit to each indoor unit, often through ceiling voids and risers. If a terminal has come loose, a cable has been damaged during other building works, or a junction wasn’t made properly during the original installation, that one unit loses its connection.
We see this particularly in offices that have had refurbishments. A ceiling contractor moves a cable, doesn’t realise what it is, and puts it back slightly wrong. Three weeks later, a unit stops working and nobody connects the two events. A methodical cable trace from the indoor unit back to the outdoor unit usually finds the problem.
Blocked Indoor Filter Restricting Airflow
A heavily blocked filter won’t normally stop the unit dead, but it can reduce airflow to the point where the evaporator coil ices up. Once ice forms on the coil, cooling capacity drops to almost nothing. The unit might cycle on and off, or just blow weakly warm air.
Pull the filters. If they’re clogged with dust and debris, clean them and check the coil behind. If there’s ice on the coil, switch the unit to fan-only mode and let it defrost before restarting in cooling mode. If the unit has been running with blocked filters for months, the coil itself may need a deep clean.
Failed Indoor Fan Motor
If the fan motor has failed or is running at a fraction of its normal speed, the unit may still be receiving refrigerant but can’t distribute the cooling. You’ll notice the unit is unusually quiet — or completely silent. The coil may ice up as a secondary effect because there’s not enough air moving across it.
Fan motors on indoor units do wear out, particularly in dusty environments or units that run continuously. The motor and fan assembly can be replaced without touching the refrigerant circuit, so it’s a relatively contained repair.
Refrigerant Distribution Issue in Branch Piping
Multi-split and VRF systems use branch distribution boxes (refnet joints or similar) to split the refrigerant flow from one set of outdoor pipes to multiple indoor units. If there’s a restriction in the branch piping serving that one unit — a kink, a brazed joint that’s partially blocked, or a faulty distribution header — that unit gets starved of refrigerant.
This is less common than communication or valve faults, but it does happen, especially on systems where the piping installation was rushed. Pressure and temperature measurements at the indoor unit versus the branch box will reveal the restriction.
What to Do Next
Start with the error code. On any VRV/VRF system, the outdoor unit’s error log is the fastest path to diagnosis. If you don’t have access to the controller or can’t read the codes, that’s where we come in.
ADK engineers work on multi-split and VRF systems across London every week — Daikin, Mitsubishi, Toshiba, Samsung, the lot. We carry PCB testers, EEV drivers, and the manufacturer diagnostic tools needed to pinpoint the fault on the first visit. One unit down doesn’t have to mean a full system replacement. Call us and we’ll get it sorted.