Daikin C4 Error Code: Indoor heat exchanger thermistor abnormality
Updated October 2026 · Sourced from Daikin documentation
🛑 Turn it off and call a licensed pro
A thermistor (temperature sensor) on the indoor heat exchanger reads outside its valid range during compressor operation. The manual lists a disconnected connector, a defective thermistor or a defective indoor board.
Is it safe to keep running? No. Turn it off and get it checked. A technician needs to check the sensor and its connector.
C4 at a glance
| Code / how it shows | Meaning | Applies to |
|---|---|---|
| C4 Shown on the remote controller (and as blinking lamps on the indoor unit) | Indoor heat exchanger thermistor abnormality | Daikin FTXS-L / FDXS-L inverter pair ductless series (09 to 36 class, models FTXS09-36LVJU, FDXS09-12LVJU) |
Check this first
Only steps a homeowner can do safely. Do not open electrical panels or the burner area.
- Switch the unit off at the breaker, wait a minute, then switch it back on and see whether the code returns.
🛑 Stop and call a licensed pro if
- A technician needs to check the sensor and its connector.
- Any code pointing to refrigerant, the compressor, high pressure or the outdoor unit board. Refrigerant work legally needs a certified technician.
- Burning smell, sparks, or a breaker that trips again after a reset.
- Water leaking from the indoor unit that does not stop after the drain is clear.
Safe first steps for any Daikin code
- Write down the exact code and how it shows (remote display or number of lamp blinks). Similar codes mean different things on different series.
- Turn the system off at the remote, switch off the breaker or disconnect for the unit, wait a few minutes, then turn it back on.
- Clean or replace the indoor unit air filters, and make sure nothing blocks the indoor or outdoor unit.
- Check that the condensate drain line is not blocked or kinked where you can see it.
- Replace the remote controller batteries if the display is dim or the unit does not respond.
Source: Daikin Service Manual SiUS091133 - FTXS-L / FDXS-L Series, Troubleshooting p.103. Codes are series-specific, so your unit's own manual is the final word. Reviewed October 2026.
Shaun Cain · Certified HVAC & Refrigeration Technician · Ask an HVAC technician
Related Daikin codes
A1 : Indoor unit PCB abnormalityA5 : Freeze-up or heating peak-cut protectionA6 : Indoor fan motor abnormalityC9 : Room temperature thermistor abnormalityU0 : Refrigerant shortageU2 : Low-voltage or over-voltage detectionU4 : Signal error between indoor and outdoor unitUA : Unspecified voltage between indoor and outdoor unitE1 : Outdoor unit PCB abnormalityE5 : Compressor overload (OL) activationE6 : Compressor lockE7 : Outdoor DC fan lock