Line 12 on my W-2 has code DD. This code means that the amount entered in the box is not taxable. Am I correct in that it should have no effect on my taxable income?

Code DD indicates the cost of coverage under an employer-sponsored group health plan. This would have no effect on your taxable income, other than the amounts you pay toward your health coverage are usually taken out of your pay before tax is calculated. This means you do not pay tax on the income used for health insurance.

Box 12 codes

• A: Uncollected social security or RRTA tax on tips reported to your employer
• AA: Designated Roth contributions under a section 401(k) plan
• B: Uncollected Medicare tax on tips reported to your employer (but not Additional Medicare Tax)
• BB: Designated Roth contributions under a section 403(b) plan
• C: Taxable cost of group-term life insurance over $50,000
• D: Contributions to your 401(k) plan

• DD: Cost of employer-sponsored health coverage. More info

• E: Contributions to your 403(b) plan
• EE: Designated Roth contributions under a governmental section 457(b) plan
• F: Contributions to your 408(k)(6) plan
• FF: Permitted benefits under a qualified small employer health reimbursement arrangement
• G: Contributions to your 457(b) plan
• GG: Income from qualified equity grants under section 83(i)
• H: Contributions to your 501(c)(18)(D) plan
• HH: Aggregate deferrals under section 83(i)
• J: Non-taxable sick pay
• K: 20% excise tax on excess golden parachute payments
• L: Substantiated employee business expense reimbursements
• M: Uncollected social security or RRTA tax on taxable cost of group-term life insurance over $50,000 (only applies to former employees)
• N: Uncollected Medicare tax on taxable cost of group-term life insurance over $50,000 (but not Additional Medicare Tax) (only applies to former employees)
• P: Excludable moving expense reimbursements paid directly to a member of the U.S. Armed Forces (not included in box 1, 3, or 5)
• Q: Nontaxable combat pay
• R: Company contributions to an Archer MSA

• S: Contributions to your 408(p) SIMPLE plan

• T: Employer-provided adoption benefits
• V: Income from the exercise of non-statutory stock options
• W: Company contributions (including employee contributions through a cafeteria plan) to an employee's health savings account (HSA)
• Y: Deferrals under section 409A on a nonqualified deferred compensation plan
• Z: Income under a nonqualified deferred compensation plan that fails to satisfy section 409A. This amount is subject to an additional 20% tax plus interest (TurboTax does not support this calculation).

Are you confusing Code DD with Code D?  

Code DD is for your information. It has no effect on your taxable income or anything else on your tax return. It's the total cost of your health insurance, including the amounts paid by both you and the employer.