W2 box 12b coded as D, employer paid health benefits, rather than DD 401k contributions. Company confirmed it is 401k. What should i code? Will this cause a problem?

The code for 401(k) contributions is "D" while the code for employer paid health benefits is "DD", as you can see from the W2 instructions below. You can enter both if you have both otherwise enter the appropriate code for the funds listed on your W2 in box 12.

D—Elective deferrals to a section 401(k) cash or deferred arrangement. Also includes deferrals under a SIMPLE retirement account that is part of a section 401(k) arrangement.

DD—Cost of employer-sponsored health coverage. The amount reported with code DD is not taxable.

You can file your return using the correct information. If the information is not correct on your W2, then your employer should issue a correction to the IRS and yourself using W2c.