Sudoku troubleshooting
What to do when you are stuck on a Sudoku
Do not stare harder at the same cells. Reset your method: verify the board, clean the candidates, change the question you are asking, and escalate one technique at a time.
The seven-minute reset
1. Verify the last few placements
Check each against its row, column, and box. If you guessed, undo to the last move you can prove.
2. Clean candidate notes
Remove digits invalidated by newer placements. Stale notes are one of the most common reasons a naked single stays hidden.
3. Scan every nearly complete unit
Look for last digits first, then cells with only one candidate.
4. Scan by digit
Pick one digit and locate every possible home across each box. This changes your view from cells to hidden singles.
5. Check box-line interactions
If every candidate for a digit in a box sits on one row or column, eliminate that digit farther along the line.
6. Check pairs
Two cells that reserve the same two digits can remove those digits from the rest of their unit.
7. Take a short break
A fresh visual pass is often more valuable than forcing a speculative move.