Stuck on a Sudoku? Test, Don’t Guess — Temp Mode

First, make sure you’re really stuck

Before testing anything, run one more honest check:

• Revisit every box you skipped. Have recent eliminations brought any digit down to one or two cells?
• Look for a digit noted twice in the same row, column or box. That means a note is wrong somewhere — fix that first, or every conclusion after it is unreliable.
• Look for pairs: two cells in a box holding the same two digits and nothing else. Those digits are locked to those cells, which removes them from the rest of the box.

If none of that moves the board, you’re genuinely stuck. Time to test.

Guessing vs. testing

Guessing writes a number and hopes. If it was wrong, the mistake hides in your board until something breaks much later, and by then you can’t tell which move caused it.

Testing writes an assumption, clearly marked as temporary, and follows it only to see whether it breaks the board. If it does, you’ve proven something with certainty. If it doesn’t, you throw the test away and you’ve lost nothing.

The test, step by step

1. Pick a pair. Find a box where a digit has exactly two candidate cells. Call them A and B.
2. Assume A. Mark the digit in A as a temporary note.
3. Follow the consequences with temporary notes only: the digit leaves B, leaves A’s row and column, and whatever that forces, follow it too.
4. Watch for a contradiction: a box where some digit has nowhere left to go, a cell with no possible digit, or the same digit forced twice into one row, column or box.
5. Contradiction found? Then A is impossible, so the digit is in B. That’s proven. Clear the temporary notes and record B as a real, single-cell note.
6. No contradiction? The test proves nothing yet. Clear it and test B, or choose a different pair.

Good pairs to test are the ones with the most consequences: a digit in a crowded row or column, or a pair whose two cells sit in different rows and columns, so either choice ripples in several directions.

Doing it in the NOTE-KU app

Tap Temp. From now on, new notes are temporary: they sit on a contrasting colour that each theme keeps distinct from your permanent notes, so an assumption can never be mistaken for a fact.

There’s a shortcut for step 2: with Temp on, if the selected digit has exactly two permanent notes in a box, tapping one of those cells lifts both notes and places a temporary note in the cell you tapped. You are now testing that option.

When the test is over, tap Clear to remove every temporary note at once. If you want the two original notes back, use Undo — or hold Undo to rewind several moves in one go.

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