Daikin indoor coil thermistor
Use for F9 or C4 sensor faults after matching the indoor head model, thermistor location, and connector pin count.
$12-$45
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Daikin Mini Split Error Codes — Quick Reference
Daikin mini-splits display error codes as alphanumeric codes on the indoor unit LED or wireless remote. Daikin uses several code families: E-codes (indoor unit faults), U-codes (system/protection faults), F-codes (sensor faults), and A-codes (safety protection). On most Daikin units, hold the CANCEL or MODE button for 5+ seconds to enter diagnostic mode and retrieve stored fault codes.
| Code | Meaning | Quick Fix |
|---|---|---|
| E1 | PCB fault — indoor unit | Replace indoor PCB |
| E3 | High pressure protection | Check outdoor coil; refrigerant overcharge |
| E7 | Outdoor fan motor fault | Check fan blade and motor windings |
| E9 | Expansion valve fault | Replace EEV (electronic expansion valve) |
| U2 | Low voltage / power fault | Check supply voltage |
| U4 | Signal transmission error | Check signal wire connections |
| U9 | Indoor/outdoor communication | Check wiring; polarity |
| F3 | Discharge pipe temperature too high | Check refrigerant charge; expansion valve |
| F9 | Heat exchanger temp sensor fault | Replace indoor coil thermistor |
| C4 | Indoor heat exchanger coil sensor | Replace indoor coil thermistor |
| A3 | Drain level protection | Clear drain line; check float switch |
| A5 | Freeze protection / high temp protection | Check airflow; refrigerant |
| H6 | Position sensor fault | Outdoor fan motor issue |
| L4 | Radiation fin temperature fault | Check outdoor board cooling fins |
Technician Tips
On Daikin mini-splits, read the code from the remote or indoor display before pulling power. Communication, thermistor, drain, and fan faults can clear temporarily after reset, so record the exact code and which indoor head reported it. Multi-zone systems should be checked zone by zone.
DIY vs Pro: Cleaning filters, checking the condensate drain, clearing outdoor-coil debris, and replacing remote batteries are reasonable owner tasks. Refrigerant, inverter-board, fan-motor, and communication-voltage diagnosis should be handled by a qualified mini-split technician.
Need Replacement Parts?
Before ordering, match the HVAC part to the model number, electrical rating, and failure symptom. Check current prices on Amazon - prices and availability change frequently.
Most Common Codes
U4: Signal Transmission Error
The indoor and outdoor units can’t communicate. On Daikin wall-mount units (FTXS, FTXB, FTXF series), the signal wire is terminal 2 and 3 between indoor and outdoor. Verify polarity — Daikin signal wiring is polarity-sensitive on many models. Swap terminals 2 and 3 if you’re unsure.
E3: High Pressure Protection
The system tripped on high refrigerant pressure. Causes: dirty outdoor coil (most common in summer), refrigerant overcharge, outdoor fan not running, or ambient temperature too high. Clean the outdoor coil with coil cleaner and rinse from inside out. If pressure is high on a clean coil, have refrigerant charge verified by a tech.
F3: Discharge Pipe Temp Too High
Compressor discharge temperature exceeded safe limits. This often means low refrigerant charge (causes the compressor to work harder with less cooling effect on the discharge line) or a failed expansion valve. A technician needs to check refrigerant pressures.
F9: Heat Exchanger Temp Sensor Fault
The indoor coil thermistor has failed. This sensor mounts on the indoor coil fins inside the wall-mount indoor unit. Access requires removing the front panel and filter. The thermistor connects via a 2-pin connector to the indoor PCB. Measure resistance — a Daikin coil thermistor typically reads ~5 kΩ at 25°C. Significant deviation means replacement is needed.
C4: Indoor Coil Sensor Fault
Similar to F9 — specifically the coil inlet/outlet thermistor. On Daikin VRV and multi-zone systems, C4 can appear when the thermistor connector has vibrated loose. Inspect and re-seat the connector on the indoor PCB before ordering a replacement part.
A3: Drain Level Protection
The drain float switch detected water overflow. Clear the drain hose (pour water through from the indoor unit outlet), clean the condensate pan, and inspect the float switch arm — it should move freely up and down. Reset by cycling power once the pan is empty.
U9: Indoor-Outdoor Communication
Typically a wiring issue on installation. Check terminal S (signal) between indoor and outdoor units, and verify ground is connected at both ends. On multi-zone Daikin systems (MXS series), verify all indoor units are wired with correct address settings.
When to Call a Pro
E9 (expansion valve fault), F3 (discharge temp high), and any code indicating refrigerant system issues require a certified technician with gauges and refrigerant certification. Operating a Daikin mini-split with a confirmed refrigerant issue will damage the compressor.