1774 Sensor head
An attempt has been made to advance or retract the sensor head while its advance/retract is not possible, or the sensor head
advance impossible state has arisen during the advance movement.
Sensor head advance/retract---- M117/M118
4->The sensor head advance/retract command is designated while the interlocked equipment is not at the retract position.
Sounds like the switch is either no good or it needs adjusting