You need Form W-2, Wage and Tax Statement, in order to file your federal tax return and your state and local returns, if applicable. If you file a paper return you have to attach a copy of the W-2. And if you file electronically you have to enter the information from the W-2. Your final paycheck may not have all the information that is contained on the W-2.
Your final paycheck will generally show your earnings and taxes withheld for the pay period and year to date. This includes your salary or wages; federal, state, and local income taxes withheld; FICA taxes withheld; and any deductions for health insurance, 401k contributions, and other deductions from your pay. These amounts on your final paycheck stub should agree with the amounts on your W-2.
But the W-2 contains other information that you need to file your return, such as the employer identification number and address. There may be amounts reported in box 12 of Form W-2 that might not appear on your final paycheck stub.
Also, Form W-2 is the control document for tax purposes. Your employer, or its payroll service provider, sends Form W-2 to the IRS, and the IRS uses the information on Form W-2 to compare with what you report on your tax return. Any differences, even though they are minor, could cause the IRS system to flag your return for review or adjustment. You could then receive a notice, which would delay your refund, if applicable. And you may have to respond to the notice with the additional information or explanation the IRS requests.
Your employer must provide you with a W-2 by January 31st and many employers issue their W-2s earlier. The IRS opens its free e-file around mid-January (January 17, 2012 for 2011 returns). So you shouldn’t have to wait long after receiving your W-2 until you can file your return. If you e-file and select direct deposit you will get your refund quicker.
According to the IRS, if you do not receive a W-2 or you receive a W-2 that you believe is incorrect, you should first contact your employer. If you do not receive a W-2 or a corrected W-2 by February 14, you can call the IRS at 800-829-1040 to ask for assistance. The IRS will contract the employer on your behalf to request the W-2 or corrected W-2.
The IRS will also send you Form 4852, which is a substitute for Form W-2. If you have not received a Form W-2 in time to file your return, you can use Form 4852 to file your return. In this case, you can use the information on your final paycheck stub to complete Form 4852. If you receive a W-2 or corrected W-2 after filing your return using Form 4852 and the information is different, you can file an amended tax return using Form 1040X.
Sources:
Form 1040X, Amended U.S. Individual Income Tax Return
Form 4852, Substitute for Form W-2, Wage and Tax Statement
Form W-2, Wage and Tax Statement
Topic 154 – Form W-2 and Form 1099-R (What to Do if Incorrect or Not Received), IRS