I thought 401k contributions lowers the amount of taxable income. But when I remove line D showing I contributed 19k to my 401k, it makes no difference to the tax owed?

Because the 401(k) contributions are not included in your gross income on the W-2 and were never taxed.

Ah of course.. thanks.  Just grasping. 2nd year in a row I have a huge tax bill even though both me and my spouse have 0 deductions and pay an extra $100 a paycheck in taxes.