Abstract: The submersible chain No.2 in the heavy water reactor's spent fuel system plays a critical role in transferring spent fuel rods from the discharge pool to the receiving pool. During routine discharges, frequent occurrences of the chain either stopping too far or too close to the mechanical stop block at the discharge end, or experiencing execution timeouts, significantly impacted operators' normal discharge procedures and intervention frequency. Through data analysis and root cause investigation, it was determined that limit switch drift was the primary cause of these issues. By adjusting the trigger mechanism of the limit switch and optimizing the switch component's preset time, the problems was effectively solved.