can i make a one time contribution to my 401k plan now to reduce my 2023 tax liability

If you are an employee and the employer provides a 401(k) retirement account. your contributions can only be made on a calendar year basis.  So, you cannot contribute to the 401(k) in 2024 and have those contributions assigned to 2023.

No.

 

Contributions to a 401(k) plan are made through payroll deductions, and can't be made after December 31, 2023.

 

Depending on your modified AGI, you may be eligible to contribute to a deductible IRA. You have until April 15, 2024 to do so. 

 

Please read this IRS document to check if you're eligible.