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.
A step-by-step reset
1. Verify the last few placements
Recheck the reasoning behind your last few entries: avoiding a repeated digit does not prove an entry is correct. Undo any trial entries you cannot justify before continuing.
2. Clean candidate notes
Remove notes contradicted by placed digits. Keep earlier logical eliminations. If your notes are incomplete, rebuild the candidates from the current grid, then reapply any eliminations you can prove.
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
Find two cells in the same row, column, or box whose complete candidate lists contain exactly the same two digits. Remove those digits from other cells in that shared unit, then scan again for singles.
7. Take a short break
A fresh visual pass is often more valuable than forcing a speculative move.