The form 8880 caps the credit to $2000 so if the 401K is $2K or more adding in more contributions will do nothing for you. https://www.irs.gov/pub/irs-pdf/f8880.pdf
Include on line 2 any of the following amounts.
- Elective deferrals to a 401(k) or 403(b) plan (including designated Roth contributions under section 402A), or to a governmental 457, SEP, or SIMPLE plan.
- Voluntary employee contributions to a qualified retirement plan as defined in section 4974(c) (including the federal Thrift Savings Plan).
- Contributions to a 501(c)(18)(D) plan.
