During the process of binding a bale of crop material with binding twine in a crop baler, malfunction of the knotter (54) is checked once upon completion of the binding sequence for one bale, and then a second time after the twine has been prepared by the knotter for formation of the next bale. The monitoring system is particularly appropriate for a double knotting mechanism in which each knotter operates through two complete operating cycles during each actuation thereof, a first cycle to complete a knot associated with the just finished bale, and a second cycle to complete a knot in two separate strands of twine to prepare the same for the next successive bale. The position of a slack takeup arm moved by the binding twine between a number of positions during the double knotting sequence is monitored by the electronic portion of the system, including a computer, so that if the takeup arm is located improperly at one of the critical checking times in the sequence, such con­dition is reported to the operator through a visual display in the tractor which tows the baler.