Check for External Triggers: Ensure that no external events or triggers are causing the state transition from the completed state to the idle state. This could include signals from other parts of the system or unintended operator actions.

Review Sequence Logic: Verify the logic in your sequence control program to ensure that the conditions for transitioning from the completed state to the idle state are correctly defined. Look for any conditions that might inadvertently be met after a certain period.

Monitor System Logs: Check the system logs to identify any events or actions that occurred around the time the state transition happened. This can provide clues about what might be causing the issue.

Adjust Timeouts: If there are any timeouts or watchdog timers in your sequence control program, ensure they are set appropriately. A timeout might be causing the state transition if the system remains in the completed state for too long.