Daikin Air Conditioner Error Code C4

C4 indicates an abnormality with the heat exchanger liquid pipe thermistor in the indoor unit. The PCB has detected either a disconnected or shorted thermistor, meaning the resistance reading is outside the valid range for a normal temperature signal. The unit stops operating normally to protect the refrigerant circuit until the fault is resolved.

No. The C4 fault is typically designed to shut down or restrict unit operation because the PCB cannot reliably monitor liquid line temperatures without a valid thermistor signal. Running the system without this protection could lead to coil frosting or other refrigerant circuit issues. The fault should be diagnosed and corrected before resuming normal use.

If the fix is simply reseating a loose connector, there may be no parts cost and only a diagnostic service call fee, typically in the range of $75 to $150 depending on your area. A replacement liquid pipe thermistor is a relatively inexpensive part, often in the $20 to $60 range, though total repair cost including labor generally runs $100 to $300. If the indoor PCB is at fault, costs can rise significantly higher due to the price of the board itself.

A knowledgeable DIYer can check whether the thermistor connector is fully seated on the PCB, which is the first recommended step and requires no special tools. Resistance testing with a multimeter after disconnecting the thermistor is also straightforward. However, if the thermistor or PCB needs replacement, working inside the indoor unit requires care around electrical components, and refrigerant-related work must be performed by a licensed technician.

A reset may temporarily clear the code and allow the unit to restart, but if the underlying cause such as a failed thermistor or loose connector has not been corrected, the C4 error will return during operation. A reset is useful for confirming whether the fault is intermittent or persistent, but it is not a repair. The thermistor and its wiring should be inspected regardless.