How to Check Sudoku Candidates Without Starting Over

When candidate notes stop making sense, clearing the entire board feels tempting. Usually the problem is local: one stale mark, one omitted possibility or one assumption that spread through nearby cells.

Audit the puzzle in small units. You can restore trust in the notes without losing the work that remains correct.

Start where the confusion began

Return to the last entry or elimination you are unsure about. Check every empty peer in its row, column and box. If the entry was sound, its digit should be absent from all those candidate lists. This local pass often reveals the single stale note causing the conflict.

Rebuild one unit completely

Choose the fullest affected row, column or box. Write its missing digits, then test each empty cell against the other two constraints. Do not rely on the existing marks during this pass. Compare the rebuilt list with the old one to find omissions and extras.

Validate pairs before using them

A naked pair requires exactly two cells in one unit containing the same two candidates. If either cell has a third candidate, or the cells sit in different units, the elimination is not justified. Restore any candidates removed because of a false pair.

Separate confirmed entries from experiments

If you entered a candidate to “see what happens”, undo to that point. Consequences of an assumption cannot validate the assumption itself. Keep uncertain digits as notes and continue searching for a placement forced by the current grid.

Resume with the simplest available step

After the audit, look again for naked and hidden singles before returning to advanced patterns. Correcting one mark often exposes a straightforward placement. This final simple scan confirms that the candidate system is useful again.