For the difficult fault cases of the test signal abnormality of CINRAD/CD, concepts and methods for standardized maintenance of large electronic equipment are introduced. In combination with the principles and signal processing of the radar system, The fault diagnosis methods such as principle analysis, leapfrog, measurement, substitution, and so on, are used to analyze and judge faults, gradually narrowing the scope of troubleshooting. The root cause of the fault is a broken signal line between receiver and monitor unit, resulting in that the control codes cannot be send to SPDT switch in the microwave module. The standardized maintenance procedures about the type of faults are proposed, and the methods and ideas of the fault diagnosis can be used as a reference in CINRAD technical support work.