S 6 (sometimes referred to as 5 6) on the display is an indication that the display is receiving power but no communication with the main board.
To resolve this issue, you will need to test the following components to verify their operation:
1. Main (Power) Board
3. Fan Board
You may test these components by swapping them with a working unit. If the unit has continues to work correctly the part you swapped is not causing your issue.
Note: to test cables move the boards from the working unit to the non-working unit.