Fujitsu air conditioner 42 error code: indoor coil (pipe) temperature sensor fault

What the 42 code means

Error 42 on a Fujitsu mini-split means the indoor heat exchanger (pipe) thermistor on the evaporator has detected an abnormal temperature, usually an open or short circuit. The lamps show 4 operation blinks and 2 timer blinks, with the economy lamp flashing.

The coil thermistor is clipped to a pipe on the indoor coil. The board uses it to stop the coil icing up in cooling and to control the fan in heating. When the reading drops out, the board shows error 42.

Fujitsu's diagnosis is the same as for the room sensor: check the thermistor for an open or short, measure its resistance and replace the controller board if the sensor tests fine.

Common causes

Listed from most to least common.

1. Indoor coil thermistor failed (open or short circuit)
2. Sensor plug loose on the indoor controller board
3. Sensor slipped off its pipe or wire damaged
4. Indoor controller board fault

How to fix the 42 code

Unplug the air conditioner or switch it off at the breaker before removing any panel or filter.

1. 1

   Step 1: Reset the power

   Turn the unit off, switch off the breaker for 5 minutes and turn it back on.

2. 2

   Step 2: Clean the filters

   Dirty filters do not cause 42, but they stress the coil. Lift the front panel, slide out the filters, rinse them and let them dry.

3. 3

   Step 3: Run it and watch

   Run the unit in the same mode. If it works for a day without 42, a glitch was likely the cause.

4. 4

   Step 4: Book a repair if it returns

   If 42 comes back, stop using the unit and call for service.

How to reset the Fujitsu air conditioner

1. Turn the system off with the remote or wired controller.
2. Switch off the breaker for the system, and the outdoor disconnect switch if there is one. Leave the power off for at least 5 minutes.
3. Restore power and turn the unit back on. The compressor normally waits a few minutes before starting, so give it time before judging.
4. If the same code or blink pattern comes back, the fault is still there. Work through the fix steps above, or call a Fujitsu contractor or other licensed HVAC technician.

When to call a technician

If 42 returns, a technician needs to check the coil thermistor's resistance and its plug, then the controller board. It is a short job for someone with a meter and Fujitsu's thermistor chart.

A typical repair for this code costs $150–$400 in the US, including parts and labor. Prices vary by area and model.

Models this applies to

• Halcyon wall-mounted mini-splits (ASU indoor, AOU outdoor)
• Halcyon floor, ceiling cassette and slim duct units (AGU, AUU, ARU)
• Halcyon multi-zone systems (RLF series)
• Ducted and ceiling-suspended units (RGLX, RULX series)

Codes can differ slightly between models. Your owner's manual is the final word and is available from Fujitsu support.

Frequently asked questions