Safety Instrumented Systems (SIS) protect major hazard facilities against catastrophic accidents. A SIS consists of hardware components and a software part - the program. Failure Mode Reasoning (FMR) is a novel abstraction technique for identifying and quantifying failure modes of SIS hardware components based on an analysis of the SIS program. In FMR, the program is divided into smaller segments, for each of which a local analysis of failure modes is carried out to identify input failure modes based on the function of the segment and a given failure mode at its output. The results of local analyses are then combined and simplified into a short list of global input failure modes. This paper explains the concepts of FMR and demonstrates its application through a real-life example. A comparison with other methods, such as Fault Tree Analysis is given in order to highlight the role of FMR in enhancing safety analysis of SIS.