What do I enter for Box 12 when my W-2 entry says,"12a-12dCode" ?

There should be a code in Box 12 along with an amount. Most common codes are D for deferred comp, W for an HSA, DD for health coverage. If your employer put an amount without a code, get a corrected W-2. If there is nothing in either box, you enter nothing.

See the full list below.

Box 12 codes are:

• A -- Uncollected Social Security or RRTA tax on tips
• B -- Uncollected Medicare tax on tips
• C -- Taxable costs of group-term life insurance over $50,000
• Code D -- Elective deferral under a 401(k) cash or arrangement plan. This includes a SIMPLE 401(k) arrangement.
• E -- Elective deferrals under a Section 403(b) salary reduction agreement
• F -- Elective deferrals under a Section 408(k)(6) salary reduction SEP
• G -- Elective deferrals and employer contributions (including nonelective deferrals) to a Section 457(b) deferred compensation plan
• H -- Elective deferrals to a Section 501(c)(18)(D) tax-exempt organization plan
• J -- Nontaxable sick pay
• K -- 20% excise tax on excess golden parachute payments
• L -- Substantiated employee business expense reimbursements (nontaxable)
• M -- Uncollected Social Security or RRTA tax on taxable cost of group-term life insurance over $50,000 (former employees only)
• N -- Uncollected Medicare tax on taxable cost of group-term life insurance over $50,000 (former employees only)
• P -- Excludable moving expense reimbursements paid directly to employee
• Q -- Nontaxable combat pay
• R -- Employer contributions to your Archer medical savings account (MSA)
• S -- Employee salary reduction contributions under a Section 408(p) SIMPLE
• T -- Adoption benefits (not included in Box 1)
• V -- Income from exercise of nonstatutory stock options
• W -- Employer contributions (including amounts the employee elected to contribute using a Section 125 cafeteria plan) to your health savings account (HSA)
• Y -- Deferrals under a Section 409A nonqualified deferred compensation plan
• Z -- Income under a Section 409A on nonqualified deferred compensation plan
• Code AA -- Designated Roth contribution under a 401(k) plan
• BB -- Designated Roth contributions under a 403(b) plan
• CC -- For employer use only
• DD -- Cost of employer-sponsored health coverage
• EE -- Designated Roth contributions under a governmental 457(b) plan