A computer program for the detection of abrupt changes in nonhardware redundant measurement signals that uses different methods of analytical redundancy is developed by the Gesellschaft für Reaktorsicherheit, Garching, Federal Republic of Germany. This program, instrumental fault detection and identification (IFDI) module, validates in real time output signals of power plant components that are scanned at a fixed rate. The IFDI module, implemented on an IBM-compatible personal computer (PC) with an 80386 processor, is tested on-line at the light water reactor off-normal behavior investigations (LOBI)-MOD2 facility in the Joint Research Centre, Ispra, Italy, during the loss-offeedwater experiment BT-15/BT-16 on November 22, 1990. The measurement signals validated by the IFDI module originate from one of the two LOBI-MOD2 facility’s steam generators. During the experiment, sensor faults are simulated by falsifying the measurement signals through electrical resistances arranged in series. Questions about the signal validation software and the steam generator’s model are dealt with briefly, while the experimental environment and the results obtained are discussed in detail.