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Fujitsu Air Conditioner 42 Error Code

Indoor coil temperature sensor error. Here is what it means and how to clear it, easiest fix first.

42
May need a pro

The sensor on the indoor heat exchanger is giving an invalid reading. Without it the unit can't protect the coil from freezing or overheating, so it stops.

Lamp pattern: on models that show faults with the indicator lamps, Fujitsu's tables for many current wall-mounted (RL/RLS/RLX, LZAH), compact ducted (RGLX) and RLF multi-zone units show this code as the Operation lamp flashing 4 times and the Timer lamp flashing 2 times, with the Economy lamp marked "continuous". Other Fujitsu units, such as universal-mount and ceiling-suspended models, use different lamps. A wired controller shows the code as a number. Listed as 42.2 in Fujitsu's FGLair app table. On the older code set this fault used 04 or 05. Fujitsu's older code set (for example older 9CQ/9RLQ/12CQ wall units, RMLQ-series multi-zone heads and some RCLX ceiling cassettes) uses different numbers for many faults, so always check the manual for your model.

Safety first: unplug the air conditioner (or switch off its breaker) before touching filters, hoses or panels. If you smell burning or gas, or see water near the outlet, stop and call a professional.

How to fix Fujitsu 42

  1. Reset the power at the breaker

    Switch the system off with the remote, then turn off its circuit breaker in your home's main panel. Wait about 5 minutes, switch the breaker back on and restart the unit. If the code returns within a few minutes, write it down for the technician.

  2. Record the details for the technician

    Write down the exact code, the model numbers from the labels on the indoor and outdoor units, and what the system was doing when the code appeared. Leave the unit switched off if the code keeps returning, so a fault doesn't cause further damage.

When to call a technician

Have an HVAC technician test and replace the indoor heat exchanger thermistor or the board.

Parts that commonly cause 42

Order parts by your exact model number so they fit.

Other Fujitsu air conditioner codes

Frequently asked questions

What does 42 mean on a Fujitsu air conditioner?

The sensor on the indoor heat exchanger is giving an invalid reading. Without it the unit can't protect the coil from freezing or overheating, so it stops.

Can I fix Fujitsu 42 myself?

Only the basic checks. You can safely try the steps above, but 42 usually points to a part or fault that needs a technician.

What if 42 keeps coming back?

A code that returns after a reset and the checks above usually means a part is wearing out. Note when 42 appears (which cycle, how often) and have your model number ready; it helps a technician bring the right part on the first visit.

Last reviewed 2026-10-19. Codes and steps can vary by model; check your owner's manual.

References