No penalty. The code P just puts the box 2a amount back into your taxable wages that is taxed as ordinary income just as it would have been if your employer had not deducted it as a 401(k) contribution on your 2017 tax return.
I doubt that It will effect your underpayment.
The IRS issued an article this morning announcing an expanded penalty waiver.
<a rel="nofollow" target="_blank" href="
https://www.irs.gov/newsroom/irs-expands-penalty-waiver-for-those-whose-tax-withholding-and-estimate...>
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**
