This paper describes a new comparison-based model for distributed fault diagnosis in multicomputer systems with a weak reliable broadcast capability. The classical problems of diagnosability and diagnosis are both considered under this broadcast comparison model. A characterization of diagnosable systems is given, which leads to a polynomial-time diagnosability algorithm. A polynomial-time diagnosis algorithm for t-diagnosable systems is also given. A variation of this algorithm, which allows dynamic fault occurrence and incomplete diagnostic information, has been implemented in the COmmon Spaceborne Multicomputer Operating System (COSMOS). Results produced using a simulator for the JPL MAX multicomputer system running COSMOS show that the algorithm diagnoses all fault situations with low latency and very little overhead. These simulations demonstrate the practicality of the proposed diagnosis model and algorithm for multicomputer systems having weak reliable broadcast. This includes systems with fault-tolerant hardware for broadcast, as well as those where reliable broadcast is implemented in software.