Abstract: A method of fault isolation comprises the steps of attaching a probe to at least one pin of each sub-circuit (32-38) of the digital electronic device (10). A predefined stimulus (20) is sent to the digital electronic device and a transmission from 0 to 1 is counted for each of the probes to obtain a first sum. A transition from 1 to 0 is counted for each of the probes to obtain a second sum. The first and second sum are then compared to a predetermined criteria and a subset of the sub-circuits, which fail to meet the predetermined criteria, is determined. From the subset a sub-circuit is identified, which failed to meet the predetermined criteria and which is closest in signal progression to the predefined stimulus (20).