This paper presents the design and implementation of a fault simulator for the TRAnsition-X fault model (TRAX for short) on a graphics processing unit (GPU). Fault dictionaries are an important aspect of on-chip fault detection and diagnosis. Generating a fault dictionary requires fault simulation with no fault dropping, requiring extensive computational resources. The inherent parallelism of the fault simulation problem maps well to the large number of concurrent threads supported by a modern GPU, and a GPU can be used to accelerate the construction of a fault dictionary. Our approach employs both pattern-parallel and fault-parallel algorithms in the GPU kernel implementations. Experiments involving various circuits, including the OpenSPARC T2 processor, demonstrate a speed-up of over 42x.