Daikin C4 Error Code

What does Daikin C4 mean?

The C4 code means the indoor unit's liquid-side heat-exchanger thermistor has reported a resistance outside the valid range — effectively open or shorted. The indoor PCB uses this thermistor to manage the electronic expansion valve and freeze-up protection, so when its reading goes out of spec, the PCB stops the unit rather than operate blind. The fault is almost always the thermistor itself, its connector, or the PCB analog input.

Symptoms

• C4 displays shortly after startup
• Indoor unit refuses to cool or heat
• Wired controller shows the indoor temperature sensor as out of range
• Error clears briefly after power cycle then returns
• Condensation or frost patterns on the coil look uneven

Common causes

• Defective liquid-pipe thermistor (open or shorted element)
• Defective connector contact between the thermistor and indoor PCB
• Cable damage from pinching, abrasion, or rodent activity
• Defective indoor unit PCB analog input
• Thermistor dislodged from its clip on the liquid pipe

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Diagnostic steps

1. Locate the liquid-pipe thermistor

   Trace the thermistor harness from the indoor PCB to the clip on the liquid (small-diameter) copper line near the coil header.

2. Reseat the thermistor connector

   Unplug the thermistor lead at the indoor PCB, inspect for corrosion or bent pins, and plug it back firmly.

3. Measure thermistor resistance

   Disconnect at the PCB and read resistance with a multimeter. Compare against the service-manual R-T table at current coil temperature.

4. Check cable for physical damage

   Run a hand along the thermistor cable looking for cuts, crush points, or chafe. Replace the thermistor assembly if damaged.

5. Confirm the thermistor is clipped to the pipe

   The sensor bead must be in firm contact with the liquid line under its insulation sleeve. Reclip if loose.

6. Replace the indoor PCB if input fails

   If the thermistor reads in spec and wiring is sound, the PCB analog channel is faulty — swap the indoor PCB.

When to call a professional

Call a licensed technician if reaching the thermistor requires dropping a cassette, opening a ducted plenum, or disassembling a concealed unit. Thermistor kits must match the model's resistance curve and connector type, and tracing harnesses through a cassette is easier with OEM documentation. If the indoor PCB needs replacement, carry-over of DIP settings, addresses, and group IDs on a multi-unit system is critical — a commissioning tech will verify the bus integrity end to end.