Dual Cultch Transmission (DCT) system has advantage both manual transmission and automatic transmission. As requirement of developing the DCT system is increasing, it is getting important to implement efficient On-Board Diagnosis (OBD) logic system to detect malfunction in vehicle to satisfy California Code Regulation (CCR 1968.2).To satisfy the CCR 1968.2, monitoring for circuit continuity and circuit faults shall be conducted continuously to detect Short to Circuit Ground (SCG), short to Circuit Battery (SCB) and Open error. In this paper, diagnostic logic for two clutches (clutch 1 and 2) and select, shift motor will be introduced to implement Transmission Control Unit (TCU) in DCT system. There are two methods to detect motor u/v/w-lines error. One is by using Application Specific Integrated Circuit (ASIC) diagnosis result and the other one is by software algorithm.Regarding diagnostic logic implemented in software algorithm, it will be derived by relationship between control duty and feedback current measured from motor u/v/w-lines and then from data measured from running vehicle, it will be shown that the diagnostic logic is reasonable to satisfy CCR 1968.2.