The excess 401(k) contribution is added back into your Wage Income for 2017, so there will be an increase in your tax liability.

Taking the excess contribution out before the due date of your return keeps you from being taxed twice on the excess contribution.  If you fail to withdraw the excess 2017 401(k) Contribution by April 17, 2018, you will be taxed on this amount now and then again when you actually take it out.

You can check out IRS Pub 525, Page 10, for more information: https://www.irs.gov/pub/irs-pdf/p525.pdf