Although deadlock avoidance issue has attracted much attention and has been extensively studied, most of the existing results assume reliable machines, which makes it difficult to apply existing deadlock avoidance algorithms to a real manufacturing system with unreliable machines. This paper presents the results to apply existing deadlock avoidance algorithms to systems with unreliable machines by analysing the robustness of the deadlock avoidance algorithms. Sequential production processes are considered in this paper and Petri Net is adopted as the tool for modelling and analysis of the sequential processes. The tolerable machine failure under which liveness property can be preserved is characterized.