Abstract: A testing instrument is especially adapted for use in locating faults in the passenger reading light and flight attendant call control systems which are used in jumbo jetliners. The testing instrument includes a selection means for designating one of a plurality of series connected passenger locations. Each passenger location includes a proximately located group of two or three passenger seats. The instrument includes indicators for registering data transmitted from the passenger location to a section timer that controls input from and output to passenger locations in an aircraft cabin on a timed, multiplexed basis. The instrument also monitors transmission of data from the section timer to the selected passenger location and includes detectors for registering faults in timing and parity pulses.