Can over contributions to 401k be self-reported or does it have to be corrected through employers?

[ears1009]

Worked for 2 employers in 2023, didn't roll-over from 1st employer.  Over contributed a minor amount in total; can this be corrected when I file my taxes or does the employer have to report it?

[Opus 17]

You must request a return of the excess amount from one of the plans.  You should then get a corrected W-2 from that employer showing the smaller deferral and higher taxable income.  

If you don't have the excess returned to you, the consequence is that it will be added back to your taxable income on your tax return (the excess is not deductible) plus you will pay tax on the money again when you retire. 

[dmertz]

If this was an excess deferral to the traditional account in one of the 401(k) plans, you must report the excess on your tax return as taxable income.  If you fail to obtain a timely corrective distribution from one of the 401(k) plans, the excess amount will be taxed a second time when eventually distributed.  Double taxation is effectively the penalty for failing to obtain the correction.