Expert 9 min read
Eleven things to check before you give up
Nearly every stuck board is a bad candidate, a missed sweep, or a technique applied in the wrong direction. This is the order to check them in.
The short version
A stuck board is information. Something specific is wrong, and there is a short list of things that are usually wrong. Work it in order, and do not skip to the expensive items because they feel more like progress.
The order below is deliberate: cheapest and most likely first, most expensive and least likely last.
1. Re-read the givens
Before anything else, check that you have not overwritten a given. It happens more than anyone admits, especially on paper and especially after an eraser has been used near a printed digit.
If the puzzle came from an app, this item is free to skip. On paper, spend the ten seconds.
2. Add up one completed unit
Take the fullest row on the board and add its nine digits. If the sum is not 45, you have a duplicate or an omission in that row.
Then do the same for one column and one box. Three sums catch the overwhelming majority of transcription errors, and they take under a minute.
3. Verify candidates against their units
For every remaining empty cell, check that none of its candidates already appears in its row, column or box. A candidate that duplicates a placed digit is stale, and stale candidates produce phantom patterns.
This is the most common single cause of a stuck board. It is also the most tedious item on the list, which is why it gets skipped.
4. Re-run the single sweep
Not a glance — a full sweep. Every digit, every box, every row, every column. Place anything you find and restart the sweep from digit 1.
The sweep, defined precisely. Most "stuck" boards fail here, because the last placement created a hidden single that nobody looked for.
5. Check both directions of box-line reduction
Pointing and claiming are easy to run in one direction and forget in the other. If you scanned boxes outward, scan lines inward. If you did rows, do columns.
Both are cheap, and both produce eliminations that no other technique sees.
6. Look for a pair you have already written
If the candidate grid is filled, re-read the units that have two identical short lists. Naked pairs that were written down and never acted on are common, because the eye slides past identical lists once they have been seen a few times.
7. Check the direction of your last elimination
This one is worth isolating, because it is the failure that produces a board which looks fine and cannot be solved.
Take the last pattern you used and ask: did I eliminate from the unit that pays, or from the unit that supplies? A naked pair pays in the unit; an X-Wing pays in the cover lines; a skyscraper pays in the cells that see both towers. Reversing any of these removes real candidates and breaks the board silently.
8. Escalate one technique level
If everything up to here passes, move up exactly one rung: pairs and triples, then fish, then wings. Do not jump to chains, and do not jump to the hardest technique you know because the board "feels" hard.
9. Look for a uniqueness argument
If the board has a rectangle with three pure {X, Y} corners, a unique rectangle will resolve it. This is cheap to check once you know the shape, and it is genuinely decisive when it applies.
10. Consider that the puzzle has two solutions
A puzzle with multiple solutions will resist many standard techniques, because they quietly assume uniqueness. If several techniques seem to fail for no reason, check whether a small rectangle could be filled two ways.
This is rare in published puzzles and common in hand-made or app-generated ones.
11. Only now, try a forcing chain
Assume a value and propagate. This always works, which is exactly why it belongs last: it is slow, it teaches you nothing about structure, and reaching for it early means you never build the pattern-recognition that makes the earlier items fast.
What to do when the list fails
If all eleven items pass and the board still will not move, the most likely explanation is that item 3 was not done thoroughly. Go back and check every candidate in the two or three most congested units by hand.
The second most likely explanation is that the puzzle is simply very hard. That happens, and the correct response is a forging chain rather than more staring.
The meta-lesson
The checklist exists because stuck boards are almost never about the hard techniques. They are about a missed sweep, a stale candidate, or an elimination applied in the wrong direction — three failures that all happen at the easy end of the ladder.
The solvers who rarely get stuck are not the ones who know the most patterns. They are the ones who run items 1 through 5 every time, without deciding in advance that this particular board must be different.
Questions people actually ask
Why does my sudoku get stuck when I have not made a mistake?
Usually you have. Most stuck boards contain a wrong candidate or a missed elimination, and the board looks correct because the error is in the pencil marks rather than in a filled cell.
How do I know if a candidate is wrong?
Check one completed unit by adding its digits. Then check that every remaining candidate is consistent with its row, column and box. A single wrong entry poisons everything downstream.
Should I restart a stuck puzzle?
Almost never. Restarting loses the information the stuck board contains. Work the checklist first — the answer is usually in the first four items.
At what point is a puzzle genuinely too hard?
When every item on this list passes and the only remaining move is a forcing chain that runs more than a few steps. That is rare in published puzzles.