Bivalue Universal Grave type 4

If the values {0} were removed from the cells {1}, the Sudoku would exhibit a Bivalue Universal Grave pattern (or BUG).

In a BUG, each value that remains in a row, column or block has exactly two possible positions in that row, column or block; and each empty cell has exactly two possible values. A Sudoku having a BUG has zero, two or more valid solutions.

Because a valid Sudoku has exactly one solution, the BUG cannot be part of it. The only possible way to avoid the BUG is if one of the two cells {2} contains one of the values {3}. If follows that the other of these two cells will contain the value {4} because this value has no other possible position in the {5}.

The values {6} can therefore be removed from the cells {1}.