Since 2010 to 2016, 401k plan has properlyreported a participant’s deferral as a Roth deferral contrbution although Employer on W-2(and employee on his 1040) has reported as a pre-tax deferral.

Will IRS accept W-2C forms(and Form 1040) from a years prior to statute of limitations. Employee will pay back taxes and penalty and interest. Is this the onlu way to avoid tainting the Roth account in the 401k PLan?

Thanks for any thoughts nd advice