Onboard Maintenance System (OMS) is a critical subsystem in modern civil aircraft that enhances overall safety, reliability, and maintenance efficiency while reducing operational costs. Fault diagnosis technology, which serves as the core component of the OMS, collects Built-In Test (BIT) information from each subsystem of the aircraft, conducts comprehensive assessments, achieves precise fault localization, and generates maintenance information that can serve as a reference for maintenance personnel. Building on this foundation, this paper proposes a general real-time fault diagnosis method for embedded systems, which includes a configurable diagnostic model and a design for the fault diagnosis function. To validate the effectiveness of the proposed fault diagnosis method, embedded real-time fault diagnosis software has been developed based on this approach and applied to the OMS system in a specific aircraft. The results demonstrate that the method can achieve rapid and accurate fault localization for the entire system and provide precise maintenance information.