Method and apparatus for detecting article engagement errors and for calibrating engagement error detection apparatus associated with a plurality of gripper arms (16) of an insertion machine (10). Gripper arms (16) at each of a plurality of insert stations S comprise a first jaw member (24) and a second jaw member (26) which selectively engage articles therebetween. Field-generating means (420) is mounted on one jaw member while a Hall Effect sensor (400) is connected on the other jaw member. The sensed flux density of the generated field is a function of the relative displacement of the first and second jaw members (24,26). A signal generated by the sensor (400) operated in a slide-by mode is related to the separation of the first jaw member (24) and second jaw member (26). If the sensor-generated signal is within predetermined limits of a stored reference signal a proper number of articles are engaged between the first and second jaw members (24,26). The reference signal is stored for each insert station S in accordance with a method and apparatus for calibrating an engagement error detector (364) associated with the station S. The engagement error detectors for a plurality of insert stations can be calibrated one-at-a-time in serial fashion or, in an alternate mode, essentially simultaneously using a master calibration switch (MCS).