Industrial vehicles incorporating obstacle detection systems are commonly employed in commercial applications. Advantageously, such collision avoidance systems should incorporate failure detection systems sufficient to make such systems fail-safe. The subject obstacle detection system includes one or more transducers 24,24' adapted to produce article detected signals in response to sensing an obstacle in front of the vehicle 12. Preferably the transducers 24,24' are pivotally mounted to the vehicle 12. The transducers 24,24' are adapted to produce the article detected signals at regular predetermined intervals, and a sensor failed signal is produced in response to the transducers 24,24' ceasing to produce the article detected signal at such regular intervals. An end of detector swing signal is produced in response to the transducers 24,24' being at a predetermined angular swing position, and logic circuitry is adapted to receive the article detected signals and the end of detector swing signals and to produce an obstacle detected signal only in response to presence of the article detected signal coincident with absence of the end of detector swing signal.