If I have any amount shown on my W-2 Box 12 like code C, DD, P, etc does it mean that none of those can be deducted?

[yatoshura]

Does it also mean that all the box12 amounts are nontaxable and not included in box1, 3 and 5 of my W-2?

Is 401K included in Box1? Can it be deducted?

[dmertz]

Your employer should have already have included or excluded from the appropriate boxes on your W-2 any amounts listed in box 12.  Specifically,

• the code C amount must be included in boxes 1, 3 (up to the SS wage base)  and 5.
• the code P amount is not included in boxes 1, 3 or 5.
• the code DD amount is not included in boxes 1, 3 or 5.

Employee deferrals to a 401(k) (code D) are excluded from box 1 but must be included in boxes 3 (up to the SS wage base) and 5.  Deferrals to a Roth 401(k) (code AA) are included in box 1, 3 (up to the SS wage base) and 5.

See the instructions for Form W-2:  https://www.irs.gov/pub/irs-pdf/iw2w3_15.pdf