One embodiment of the invention provides apparatus and a method for testing a transmission path across one or more printed circuit boards. According to the method, a test signal is presented at a first location on the transmission path. The test signal is generally low frequency compared to normal data communications on the transmission path. A pickup line is capacitively coupled at a second location to the transmission path. The pickup line is monitored with a detector to see whether or not the test signal is present. If the test signal is not present, it is determined that there is a fault on the transmission path between the first location and the second location.