Daikin Heat Pump Error Code C4

C4 indicates an abnormality in the indoor heat exchanger thermistor 1 (R2T), which monitors the liquid pipe temperature on the indoor unit. The fault is triggered when the indoor PCB detects a resistance value from R2T that falls outside the acceptable range, pointing to a disconnected, shorted, or failed thermistor. The unit shuts down to prevent operating without accurate temperature feedback.

No. The C4 error causes the unit to shut down specifically because it cannot reliably monitor liquid pipe temperature without a functioning R2T thermistor. Attempting to override or reset the unit repeatedly without resolving the underlying fault risks operating the system without proper refrigerant flow control, which can damage the compressor or other components.

Repair costs vary depending on the root cause. If only the R2T thermistor needs replacement, parts and labor typically fall in the range of $100 to $300. If the indoor unit PCB is at fault, the cost can rise to $400 to $800 or more depending on the model and labor rates in your area. A diagnostic service call is usually needed first to confirm which component has failed.

Some basic checks are within reach for a knowledgeable DIYer, such as visually inspecting the thermistor connector and wiring harness for obvious damage or a loose connection. However, accurately diagnosing a thermistor fault requires a multimeter and access to the resistance reference table for the R2T sensor. Replacing the thermistor or PCB involves working inside the indoor unit and is best handled by a qualified technician.

A power reset may temporarily clear the fault display, but if the R2T thermistor is physically damaged, shorted, or disconnected, the C4 code will return once the unit attempts to operate and the PCB detects the out-of-range signal again. The underlying sensor or wiring issue must be repaired before the fault will stay cleared.