Scroll back for the givens grid. After the fourth 4-fill, we never get back to the three unresolved 3-fills in the bypass trace.
Here is the grid, just before SE6 (halfway down) wipes out the naked pairs.
The Sysudoku basic level close down is complete. Many basic level example traces are reached by scrolling back. For each example, the givens grid is one post further back.
Earlier basic level examples, and advanced examples, can be located by date on Titles, and found by the monthly archive.
The weekly posts are extended to add in some examples of boxline exclusion in Sysudoku Basic.
Here is 1505 from Sudoku Puzzle Book for Adults, Extreme Hard by Markus Mangold, Gingen & Sons, Germany, available on Amazon. Note there’s one 3 and no 4 givens. Dust off the boxline page in the Guide. Boxlines, including slink and aligned triple pointers, help in unbalanced puzzles of this type.
Systematic Sudoku 