It is an extremely crucial task for oil and gas industry to assess how reliable gas quality analyzing systems are. The traditional approaches to assess the reliability of this class of systems, due to a number of conditions, do not seem to be applicable. To assess the reliability of such systems, we propose a probabilistic method to determine a distribution function of operating time, a reliability function, and mean time to failure. We build a structural diagram of the system, obtain basic formulae to determine the required functions and reliability indicators.We investigate various scenarios of system failure depending on the number of parameters for which accuracy decreases. For each of the scenarios, we obtain formulae to describe the required functions and reliability indicators.