Daikin U4 error code: indoor-outdoor transmission error

What Does Error U4 Mean?

For the documented Daikin SkyAir RZR-P and RZQ-P(9) systems, U4 is a transmission error between indoor and outdoor units after normal communication is absent for the specified interval. Applies to: SkyAir RZR-P and RZQ-P(9) This definition is taken from the SkyAir service manual covering RZR-P and RZQ-P(9) equipment and the indoor units listed with it. Daikin uses structured code letters across product lines that share little else, so a U4 on VRV, Altherma, or a residential split needs the manual for that line rather than this page. Owner-safe checks: Write down the full controller code and every connected indoor and outdoor model number. | Check only normal user-accessible power controls and note whether other units on the same system operate. Qualified technician: Transmission wiring, fuses, fan motors, and PCBs require qualified electrical diagnosis. | Multi-unit SkyAir systems should be diagnosed as a network rather than by replacing the first board named.