I over contributed to my 401k in 2016. What do I report in 2016 without the 1099-R? What boxes should I not miss filling out?

[RichardK]

If you work for the same employer during 2016, then the employer should correct the error to avoid the plan from being disqualified. Click here for more information from the IRS.

Per the IRS website:

Employees whose elective deferrals exceed the limit must report the excess as income on their tax returns for the calendar year the deferral was made and on their tax returns for the calendar year when the excess amounts are withdrawn.

The only way to correct the mistake, avoid double taxation and potential plan disqualification is to have the excess amount, plus earnings, refunded to the employee by the tax-filing deadline for the year.