Safety Instrumented Systems (SIS) are electric/electronic/programmable electronic safety related systems which are intended to achieve or maintain a safe state for the Equipment Under Control (EUC), in respect of a specific hazardous event. A well designed system can increase plant availability by reducing the number of spurious « Trips » caused by an SIS that fails to properly evaluate a safety situation and unnecessarily shut down a process. Data suggest that over 90% of problems in SIS can be attributed to the sensors and final elements. This paper presents the results of a simulation and experimental study based on fault diagnostic of SIS components (especially sensors). The EUC is a furnace in gas process industry. It has been shown that adding a second sensor to the SIS may improve its performance (Safety Integrity Level: SIL).