By keep asking “what happened to the part that was connected between normal and fault?” The basic problem of fault diagnosis is put forward. Based on the parameter analysis, the most basic problem of fault diagnosis is pointed to the generalized stiffness looseness through the congenital underlying logic presupposition of the fault. The relationship between generalized looseness, part fault and system fault is analyzed by analytic method, which further proves that looseness fault is the most basic problem in fault diagnosis.Finally, the essential priority of looseness fault in fault diagnosis is expounded.The essential priority of loosening fault in fault diagnosis explains the basic problem of fault diagnosis and provides a scientific basis for the systematic development of fault diagnosis.