Did you enter your contribution by entering information from your W2 Box 12, code D? Or did you enter that $600 somewhere else?
The 401(k) plan contributions you elect to make come directly out of your salary. Since the contributions are made with pre-tax dollars, your employer does not include these amounts in your taxable income for the year. At the end of the year, when you receive your W-2 form that shows your earnings, you will notice that your wages subject to federal income tax are lower because of your 401(k) plan contributions.
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