Abstract: A probe for sensing the direction of flow of an injected current pulse along a conductor of a circuit under test. The probe is linked to automatic test equipment (ATE), and is triggered to inject the pulse during a test sequence at a step previously found by the ATE to establish a faulty state in the circuit so that the faulty one of several components connected to a circuit node can be identified. The probe injects an approximately triangular-waveform current pulse which has a steep rising edge and a less steep falling edge, thereby inducing a voltage level and thus permits discrimination of the direction of current flow. To detect low level current pulses in the presence of masking noise, the output of the probe is connected to the series combination of a filter, an integrator and an A/D converter. The circuit under test is set to its faulty state, and then the probe is repeatedly operated, first with the injection of the current pulses inhibited and then with current pulses being injected.