Engine failsafe warning is coming on and shutting down the engine

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While driving at slow speeds, the engine failsafe warning will sometimes come on and as a result, the engine will cut off. It initially happened four months ago but has been happening more often recently. To resolve the problem, I turn the engine off, remove and then reinsert the key and the engine starts right back up. When it first happened I took the vehicle to the dealership and they ran a diagnostic test on the engine that revealed no problems. Was advised that the sensor that controls the failsafe is located in the throttle body and that it might be going bad however, was advised that you cannot replace just the sensor but would rather need to replace the entire throttle body because the sensor is part of the throttle body. Is that correct? Any suggestions?