23Fujitsu · ASTG09 / ASTG12 / AOTG09 / AOTG12 KMTC · Mini Split

Fujitsu ASTG09 / ASTG12 / AOTG09 / AOTG12 KMTC error code 23

Combination error

Needs attention soon

Also written as combination, incompatible pairing, code 23

Quick answer

On Fujitsu ASTG09 / ASTG12 / AOTG09 / AOTG12 KMTC, 23 is a fault. It means combination error. A first thing to check: An indoor and outdoor unit pairing that is not a supported combination.

Applies to: Fujitsu ASTG09 / ASTG12 / AOTG09 / AOTG12 KMTC; Fujitsu GENERAL AIRSTAGE KMTC wall split codes, indoor and outdoor · Mini Split
Type of code: Fault; Urgency: Needs attention soon
First safe check: Confirm the indoor and outdoor model designations are a supported pairing for this series.

Checked 2026-09-26

What 23 means on this model

The combination of equipment connected to this system is not one the unit will run. This is a configuration code rather than a failed component, and it is a code to read alongside 32 and 62, which are the ones that point at a board reporting an identification the unit does not recognise. The unit will not start because it has not been told, or cannot work out, what it is connected to, and a mismatch here will not clear by replacing any part.

Symptoms you will notice

Code 23 on a newly installed system
The system will not start after an indoor or outdoor unit was changed
The fault appeared after adding or removing an indoor unit
The system worked before a swap and does not now

Common causes

The code tells you what the control detected, not which part failed. These are the causes we see most often, roughly in order of likelihood:

An indoor and outdoor unit pairing that is not a supported combination
An indoor unit swapped for a different model without reconfiguring
More indoor units connected than the system supports
A field setting that does not match the equipment installed
A failed indoor or outdoor PCB reporting a model identification it does not expect

What you can safely check

These are homeowner-level checks. None of them need you to open a live electrical compartment or work on gas.

Confirm the indoor and outdoor model designations are a supported pairing for this series.
Check whether any unit has been recently added, removed or replaced, because that is when this code appears.
Check the field settings against the equipment actually installed.

Do not do these

×Do not swap indoor or outdoor PCBs to clear a combination code. Code 32 and code 62 are the ones that point at board identification, and combination is a configuration problem that a board swap will not fix.
×Do not force the system to run on an unsupported pairing. The refrigerant control is sized for the connected load.

When to call a technician

Book a technician, do not keep resetting it

This fault will not clear on its own. Every power cycle gives the control one more failed attempt, and repeated attempts are what turn a cheap part replacement into a seized or contaminated system.

The refrigerant circuit is sealed and legally requires licensed handling. Opening it without certification releases refrigerant and can put the whole system in violation of regulation, so a low-pressure or high-pressure fault on this equipment is a call-out, not a weekend job.
If the appliance has stopped mid-cycle, is making a noise you have not heard before, or smells burnt, stop reading here and call. Those are symptoms of something the fault code does not describe well enough.

A commissioning technician job. The compatibility information is in the service documentation.

What it costs to fix

Parts

None expected. This is a configuration code.

Labor

Two to four hours to correct the combination or the settings.

DIY or technician