Is it correct for KIA k3 to clean the throttle and power off first?

Correct. If the power supply is not stopped, the computer version will think that the communication with the throttle is suddenly interrupted, which will lead to the occurrence of fault codes and even affect the idle speed of the engine. By the way, it is best to match the throttle after cleaning, because it is an electronic throttle, otherwise there will be surprises waiting for you. Thank you!