Identifying and Rectifying the Potential Faults in the Probabilistic Metric Formula in ISO 26262

ISO 26262 is the functional safety standard for automotive electrical and electronic (E/E) systems. Part 5 of the standard addresses the hardware design and verification phase of the safety lifecycle in automotive system development. It introduces a probabilistic metric for random hardware failures (PMHF) as a numerical safety target. Our research, initiated in 2017, has led to the development of a new PMHF formula, referred to as AF2, which we introduced in our RAMS 2020 paper. This paper scrutinizes the derivation process of the PMHF formula in ISO 26262:2018, termed SF2, and identifies 11 potential faults arising from four different causes. These faults significantly influence the accuracy of the PMHF calculations. Importantly, after these faults are rectified, the PMHF formula aligns with our proposed AF2 formula, substantiating the validity of AF2. Given the limited research on the PMHF formula, these issues in the derivation process of the formula may have been overlooked. We anticipate that this paper will contribute to enhancing the PMHF formula in ISO 26262 and, by extension, improve design reliability in the automotive industry.