How would you make a new 401K contribution? They have to be deducted from your paychecks. It is too late to increase your deduction for 2023. Anything deducted in 2024 will go on your 2024 W2 next year.
And if you can write a check to the 401K it would be non deductible and not pretax. And isn't reported on your tax return. The plan would keep track of it and when you take a distribution part of it will come out tax free,
