Fujitsu ASTG09 / ASTG12 / AOTG09 / AOTG12 KMTC error code 73
Outdoor heat exchanger liquid temperature sensor error
Quick answer
On Fujitsu ASTG09 / ASTG12 / AOTG09 / AOTG12 KMTC, 73 is a fault. It means outdoor heat exchanger liquid temperature sensor error. A first thing to check: A failed outdoor heat exchanger outlet thermistor, designated ThHO.
- 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 16.1 kOhm at 0 C, 6.0 kOhm at 20 C and 3.8 kOhm at 30 C.
What 73 means on this model
The thermistor on the liquid line at the outdoor heat exchanger outlet has failed, so the unit has stopped. The documented test uses the outdoor pipe thermistor curve, which is the steepest of the three outdoor sensors: 16.1 kOhm at 0 C, 6.0 kOhm at 20 C, and 3.8 kOhm at 30 C. Those much lower numbers are the giveaway that you are on the pipe sensor rather than the discharge or outdoor temperature sensor, and mixing the three curves up is the most common mistake when testing them.
Symptoms you will notice
- Code 73 and the system will not start
- The system will not hold capacity
- The system performs poorly in both heating and cooling
- The fault followed work on the outdoor unit
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 outdoor heat exchanger outlet thermistor, designated ThHO
- A short or open in the wiring to the sensor
- A disturbed connector
- A clip that has come loose from the liquid line
- A sensor whose resistance is out of range
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 16.1 kOhm at 0 C, 6.0 kOhm at 20 C and 3.8 kOhm at 30 C.
- Check that the sensor is still clipped to the line and that the wire is undamaged.
- Check the connector at the sensor.
Do not do these
- ×Do not use the wrong resistance curve when testing. The pipe sensor reads roughly a tenth of the discharge sensor at the same temperature, and swapping the two curves produces a false pass.
- ×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.
What it costs to fix
Parts
Outdoor pipe thermistor $30 to $90.
Labor
One to two hours to test and replace.
DIY or technician
A technician job.
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
- Models in scope
- Fujitsu ASTG09 / ASTG12 / AOTG09 / AOTG12 KMTC
- 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.