This paper establishes a formal modelfor hybrid diagnosis, novel features including: (1) It provides aunified theoretical framework for utilizing device models and heuristicsin diagnosis, which naturally integrates all the important components ofdiagnosis --- the structural and behavioral description of devices,fault modes, the lower and upper fault bounds, fault possibilities andheuristic rules --- into a diagnostic system. Device models predictoutputs from inputs, heuristic rules infer the possibilities of certaincomponents being faulty from symptoms, and yet the combination of bothconstrains each other for us to reduce the hypothesis space. (2) Itpresents a typical way of modeling behavior of devices, to which the keyis the introduction of I-O functions with indefinite inputs/outputs. (3)It can easily be implemented over a forward-chaining inference engine.