Fujitsu ASTG09 / ASTG12 / AOTG09 / AOTG12 KMTC error code 42
Indoor heat exchanger middle temperature sensor error
Which models this applies to. This page covers the Fujitsu GENERAL AIRSTAGE KMTC wall split codes, indoor and outdoor.
What is specific to the ASTG09 / ASTG12 / AOTG09 / AOTG12 KMTC platform
ASTG09KMTC and ASTG12KMTC indoor units with AOTG09KMTC and AOTG12KMTC outdoor units on R32. A compatible non-polar wired controller shows the code directly on its display. The supplied wireless controller has no screen and shows the same codes as a flashing pattern on the indoor indicator lamps instead. The service reference gives a separate main PCB, control unit and inverter assembly identification for each of the two capacities, so a board from the wrong capacity is a recognised cause of codes 32 and 62.
Your control board prints the code as text, for example F01 or EE3, so you read it off the display rather than counting blinks. The printed number is the fault itself, not a position in a table.
Quick answer
On Fujitsu ASTG09 / ASTG12 / AOTG09 / AOTG12 KMTC, 42 is a fault. It means indoor heat exchanger middle temperature sensor error. A first thing to check: A failed indoor heat exchanger middle thermistor.
- 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: Book a technician
- First safe check
- Measure the sensor and compare it to 176.0 kOhm at 0 C, 62.9 kOhm at 20 C and 39.6 kOhm at 30 C. If you are getting numbers near 33.6 kOhm instead, you are on the room sensor, not this one.
What 42 means on this model
The thermistor at the middle of the indoor heat exchanger has failed, so the unit has stopped. This sensor, designated ThHM, is what the control uses to work out how much heat or cold the coil is actually moving, and without it the unit cannot decide how long to run or where to modulate. The documented test is a resistance check against the indoor pipe thermistor curve, which is a different curve from the room sensor: 176.0 kOhm at 0 C, 62.9 kOhm at 20 C, and 39.6 kOhm at 30 C. The much higher resistance at every point is what tells you which sensor you are looking at.
Symptoms you will notice
- Code 42 and the unit will not heat or cool
- The system stops shortly after starting
- The unit runs erratically and will not hold setpoint
- The fault followed a coil or fan service
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:
- A failed indoor heat exchanger middle thermistor
- A short or open in the wiring to the sensor
- A disturbed connector
- A sensor clip that has come loose from the coil
- A sensor whose resistance is out of range for the temperature it is reading
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.
- Measure the sensor and compare it to 176.0 kOhm at 0 C, 62.9 kOhm at 20 C and 39.6 kOhm at 30 C. If you are getting numbers near 33.6 kOhm instead, you are on the room sensor, not this one.
- Check that the sensor is still clipped to the coil and that the wire is not pinched or cut.
- Check the connector at the sensor.
Do not do these
- ×Do not clip the sensor back onto the coil loosely or leave it hanging, because the control treats this reading as a measure of what the coil is doing.
- ×Do not work on the connector with the power switch on.
When to call a technician
Worth scheduling, and worth your own checks first
You can usually keep living with this for a short while, but it will not fix itself. Do the safe checks below first, because a tripped switch or a blocked airflow is a five-minute fix that would otherwise cost you a service call.
- 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 technician job once the coil is involved.
What it costs to fix
Parts
Indoor coil thermistor $25 to $80. Indoor heat exchanger $250 to $600 if damaged during removal.
Labor
One to two hours to test and replace.
DIY or technician
A technician job once the coil is involved.
Typical US part and labor ranges, 2026. They vary by region, model and contractor, so treat them as a planning figure, not a quote.
Scope of this code
- Code table
- Fujitsu GENERAL AIRSTAGE KMTC wall split codes, indoor and outdoor
- How this model reports the code
- Printed on the control display
- Models in scope
- Fujitsu ASTG09 / ASTG12 / AOTG09 / AOTG12 KMTC
- How this platform differs
- ASTG09KMTC and ASTG12KMTC indoor units with AOTG09KMTC and AOTG12KMTC outdoor units on R32. A compatible non-polar wired controller shows the code directly on its display. The supplied wireless controller has no screen and shows the same codes as a flashing pattern on the indoor indicator lamps instead. The service reference gives a separate main PCB, control unit and inverter assembly identification for each of the two capacities, so a board from the wrong capacity is a recognised cause of codes 32 and 62.
- Checked
- Confirm on your own unit
- Find the model number on the data plate, then check this code in the service manual for that model. Manufacturers revise code tables between model years, and a revised board can renumber a fault.