If you find you’ve overcontributed to your 401(k), contact your employer or plan administrator as soon as possible. Tell them you’ve made an excess deferral and the amount.
It’s best if you catch this error before April 15 in the year following the overcontribution.
Once you’ve alerted your plan administrator, processing your overcontribution error may take a bit of time. The plan administrator is required to distribute the excess deferral and earnings attributed to those funds by a certain date laid out in the 401(k) plan. Earnings are calculated based on the total change in value of the account since the overcontribution.
It’s important that this process be completed before April 15 of the year after the overcontribution is made, so you need to check all your W-2s and records as soon as you get them to make sure you have time to correct the error if you overcontributed.