An optoelectronic sensor (10) is provided with a light transmitter (12) for transmitting a light signal (16) into a monitoring area (18), a light receiver (26) for receiving and converting the light signal (22) remitted or reflected from an object (20) in the monitoring area (18) into a reception signal, a switching output (30) for the output of a switching signal when the reception signal meets a switching criterion (E), a pre-failure alarm output (32) for the output of a pre-failure alarm signal, and an evaluation unit (28) configured to detect a contamination state that is present when the reception signal meets a contamination criterion (C) but not the switching criterion (E) and to output a pre-failure alarm signal when a contamination period during which the contamination state extends for more than a tolerated period (TPFA,ON) and/or when a contamination state occurrence number that is incremented upon each entry of the contamination state is larger than a tolerated number. The evaluation unit (28) is further configured to add up the contamination period also across several discontinuous periods of the contamination state or to count the contamination state occurrence number also across periods during which neither the switching criterion (E) nor the contamination criterion (C) are met, respectively.