84Fujitsu · ASTG09 / ASTG12 / AOTG09 / AOTG12 KMTC · Mini Split

Fujitsu ASTG09 / ASTG12 / AOTG09 / AOTG12 KMTC error code 84

Current sensor error

Needs attention soon

Also written as current sensor, overcurrent, compressor current, code 84

Quick answer

On Fujitsu ASTG09 / ASTG12 / AOTG09 / AOTG12 KMTC, 84 is a fault. It means current sensor error. A first thing to check: A failed current sensing circuit on the inverter board.

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: Measure the compressor winding resistance and compare it with the documented 1.975 Ohm at 25 C across U to V, V to W and U to W. All three should match closely, and one that reads very different from the other two points at a winding rather than at the sensor.

Checked 2026-09-26

What 84 means on this model

The unit has lost the ability to measure compressor current, so it has stopped. Current sensing is how the control knows what the compressor is doing, and it is what protects the compressor, detects a closed valve, and feeds the inverter. A failure here often travels with a genuine inverter fault or a compressor problem rather than being a failed sensor on its own, so the current sensor, the inverter, and the compressor are checked as a set. The documented test on the compressor side is a winding resistance check.

Symptoms you will notice

Code 84 and the system will not start
The system stops shortly after the compressor starts
Current related codes appear alongside it
The fault appeared after a power interruption

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 current sensing circuit on the inverter board
A compressor winding that has shorted or gone open
A loose or corroded compressor connector
A failed inverter assembly
A wiring fault in the current path

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 compressor winding resistance and compare it with the documented 1.975 Ohm at 25 C across U to V, V to W and U to W. All three should match closely, and one that reads very different from the other two points at a winding rather than at the sensor.
Check the compressor connector for a loose or corroded pin.
Check the other codes on the system, because an inverter fault often produces this alongside it.

Do not do these

×Do not replace the inverter board before measuring the compressor windings, because a shorted winding will destroy the new board.
×Do not keep restarting, because the control is protecting the compressor from a current condition it cannot identify.
×Do not work on the compressor terminals with the power switch on.

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 licensed technician job.

What it costs to fix

Parts

Inverter assembly $600 to $1,400. Compressor $1,400 to $3,000. Compressor connector $30 to $90.

Labor

Three to eight hours depending on whether the cause is the sensor or the compressor.

DIY or technician