401k w2 box 12 code D

Contributions to an employer sponsored 401(k) are not reported on a tax return.  They are pre-tax contributions.

401K code D.  It comes out of your checks pretax and has already been deducted from the wages in box 1 and you can't deduct it again.  Notice that box 1 should be less than boxes 3&5.

The value in box 12, code D, just defines how much of the wage difference for box 1 was from pre-tax 401k contributions.