This article deals with a theory-based investigation of the diagnostic problem-solving process in professional contexts. To begin with, a theory of the diagnostic problem-solving process was developed drawing on findings from different professional contexts. The theory distinguishes between four sub-processes of the diagnostic problem-solving process and includes several hypotheses. According to the theory, the quality of the sub-processes "representing information" and "testing hypotheses" causally influences the diagnostic problem-solving success. Additionally, the theory suggests that the influence of "testing hypotheses" on the problem-solving success is higher than the influence of "representing information". Moreover, the theory assumes that the influence of the quality of "representing information" on the success is mediated by the quality of "testing hypotheses". These hypotheses were examined in the context of car mechatronics using diagnostic problems of the car sector, a computer-based assessment and a sample of car mechatronic apprentices (N = 339). To operationalize the sub-processes' quality, observable critical behavior was theoretically identified and extracted from computer-generated log-files. The empirical results were largely in line with the hypotheses and indicated a (very) first corroboration of the theory in the context of car mechatronics. The theory could be helpful to investigate and teach diagnostic problem solving in different professional contexts. Further studies, however, should scrutinize whether the theory applies to other studies and professional contexts.