Abstract: Recently,many researchers have noticed the importance of applying the concept of causation to model based diagnosis.However,their researches are limited to simple causal theory.Here proposed is the concept of general causal theory which contains more information.It is pointed out that the diagnosing space of a general causal theory is smaller than or equal to that of the corresponding simple causal theory.Also given are the concepts of model based diagnosis and kernel model based diagnosis when the model of the system to be diagnosed is general causal theory.Then it is demonstrated that the direct relationship between kernel model based diagnosis and the prime implicants/implicates.Therefore,the theoretical results in the paper can be linked with implementation.Furthermore,it is pointed out that the characterizations of kernel consistency based diagnosis and kernel abductive diagnosis of general causal theory are the only two special cases of the characterization.