A natural gas fired industrial oven exploded during an attempted restart. The explosion was caused by the accumulation of natural gas during the hours preceding the restart. The oven had a fully-automated control system, and the start, shutdown, and restart were controlled by process controls and a flame safeguard system. A malfunction of the flame detector allowed the automated controls to complete the start sequence and establish normal operating status without a flame present in the burner. The ultraviolet flame sensor gave a false positive signal, which allowed unburned natural gas to enter the oven. This investigation explored the cause of the incident and the failure modes of the flame detector. Recommendations are included for preventing this type of accident. (C) 2011 American Institute of Chemical Engineers Process Saf Prog 30: 377-380, 2011