In the process of fabricating high density mounting boards, locating faults in boards rejected in a functional test requires special circuit knowledge and skills, so it has been becoming more and more difficult to find the faults in a limited time. To address this problem, we have developed an Auto-Fault-Locating-System for mounting boards using a simple comparison technique and faulty net tracing algorithm. In the system, net-propagated faults are detected by comparing pulse-counts and voltages in good boards and faulty ones under full power supply. Faulty nets, which include the fault origin, are detected by tracing backward via a faulty propagation path using the input-output attribute of terminals. Measurements require 2.5 seconds per net (i.e., about 40 minutes for 1,000 nets), and searching faulty nets requires 10-20 seconds. The system successfully located the fault origin in 22 out of 25 boards rejected in a functional test. The located fault origins were found to be IC problems, pattern errors, open terminals and shorts between nets.