This paper mainly fills the gap of diagnosis test for non-observable common variables or high cost of testing common variables. The concept of replacement test is presented as a new alternative to diagnosis test. The concept of relevant replacement test is proposed for some observations, and makes best use of the effects of component replacement upon the observations of the system being diagnosed to characterize the discrimination of candidate diagnoses, and the generation of new conflicts. Based on it, the concept of replacing decomposition for diagnostic problems is proposed by virtue of the characteristics of replacement, and the decomposition of the system being diagnosed through replacing the components in the intersection set of some subsystems with normal components directly is investigated. In non-observable common variables or high cost of testing common variables, the results in this paper can improve the adaptation and the effectiveness of diagnosis tests, reduce the cost of the test, and provide a theoretical basis for decomposing the system being diagnosed.