Wrong 401k contribution on W2. What should I do?

[kawhiet]

I noticed that my employer made a mistake on Box 12 Code D of my W2 (Elective Deferral Under 401-k), which put me over the annual contribution limit and I would actually be under. For simplicity, let's say I contributed $15,000 but they somehow had me at $22,000 ($7,000 more than I did, and $1,500 over limit). The $7,000 never hit my 401-k account or my paycheck. I told my employer already, but given that the deadline is tomorrow I wonder what action should I take: 

1) File tax as is, and file a corrected W2 later when I receive it

2) File a 1099-R and treat the $7,000 as a withdraw

3) File an extension (form 4868)

4) Other (please specify)

[DanaB27]

It's probably best to file an extension and wait for the corrected W-2.

Please remember, an extension does not give you extra time to pay your taxes, which are due on April 18. Please see IRS Make a Payment

Please see How do I file an IRS tax extension? for instructions.