Generally you need to add your retirement plan contributions back into your income for FAFSA purposes. There is an exception for mandatory contributions such as in many public sector retirement plans. That’s because those are not discretionary, unlike contributions to a 401k or 403b.
How do you know if your contributions need to be reported? First, are they mandatory or elective? Some public retirement plans such as 457 plans automatically deduct a fixed percentage of employee pay as an employee plan contribution; there is no opt-out option. Assuming they are mandatory, you need to look at where they’re reported on your W2. If they’re reported in Box 14, they do not need to be reported on the FAFSA. (If they’re mandatory and reported in Box 12, ask your employer why.)
Note that mandatory contributions really only exist in some public sector plans. Automatic enrollment in a 401k does not constitute a mandatory contribution because you can always opt out.