An expert system for PCS-1900 radio frequency module test failure diagnostics that incorporates fuzzy logic has been implemented in the Wireless Networks manufacturing facility of Nortel in Research Triangle Park, North Carolina. Since the relations between the results of parametric test measurement and failure modes are neither certain nor definite, it is advantageous to employ fuzzy logic in building a knowledge base system. Instead of providing a definite conclusion that specifies a specific failure mode, this system gives a degree of failure for each mode, indicating the possibility of that mode being faulty.