BUG+1

An almost-all-bivalue grid forces its one tri-value cell.

Learn the BUG+1 Sudoku technique: when every cell but one is down to two candidates, the odd cell must take its thrice-seen digit.

How to use it

1. Spot the bivalue grid. Notice that every unsolved cell has exactly two candidates, except one cell that has three.
2. Avoid the deadly pattern. A fully bivalue grid (a BUG) would have two solutions — which a real puzzle never has.
3. Place the odd digit. In the tri-value cell, place the candidate that appears three times in its box (an odd number).