Abstract: An uncoupling tool has a two-module design: one module being a driver assembly for developing the uncoupling movement and the other module being a probe assembly for verifying the uncoupled condition. The probe assembly attaches to the threaded end of the piston tube with its probe extending inside the piston tube. The driver assembly attaches to the CRD ring flange. The probe assembly detects the ring magnet in the drive piston using analog position sensors which operate on the principle of the Hall effect and solid state circuitry. A top sensor provides a signal indicating that the control rod is in its "full out" position; a bottom sensor provides a signal indicating that the control rod is uncoupled; and an intermediate sensor provides a signal indicating that the control rod is in a transition position between the "full out" and uncoupled positions. The probe incorporates spacers which focus the magnetic flux onto the sensors.