The fault detection and diagnosis system, which uses dedicated interface adapter unit and PXI interface modules to perform signal excitation and parameter testing, is introduced in this paper. Its firmware consists of PXI-bus control computer, display control unit, dedicated interface adapter unit, connection cable and power supply unit. The dedicated interface adapter unit (IAU) is the core of hardware platform, and is composed of chassis, I2C-bus data acquisition board, adapter and military aviation sockets. It carries out analog to digital, digital to analog, digital I/O transformation as well as serial communication and CAN-bus communication. The software of the fault detection system is of hierarchical modular structure with integration of system management control, fault detection and circuit hardware driver modules together with repair and diagnosis database. The software provides functions of human-machine interaction, equipment fault detection, fault diagnosis and analysis, repair guidance and data storage. Therefore, this system can implement fault detection of hydraulic excavator on replaceable circuit board and block of the hydraulic system or electrical system. And it can help equipment repairmen and operator perform quick repairs and maintenance to the electrical system and hydraulic circuit of the excavator.