Deduction of 401k Contribution for Schedule C Employee

Hello:

I have a 401(k) Plan sponsored by a Sole Proprietorship that terminated on 12/31/2018.  As part of the process to get all of the participants paid out, it was determined that none of the owner's 2017 401k deferrals were deposited to the Trust.  The owner deducted $24,000 on their individual tax return, and it was reported as a contribution on the 5500 form (and included in all of the necessary testing).  The accountant is asking whether it is still deductible on the 2017 tax return if the deposit is made today (2019).  I'm thinking, from the plan's standpoint, this will be treated as a late contribution and applicable earnings will be deposited along with the $24,000 and the tax return stands as is.  His other option is to amend his 2017 tax return to remove the $24,000 deferral.  Any thoughts?  Let me know if you need more details to provide your feedback.    

I think it is pretty clear that it is not deductible in 2017 if not deposited by the due date of the return.  But I know of a lot of accountants that would not want to be bothered amending a return and would take their chances, and I don't see it as the plan's (or TPA's) problem as far as that goes.  If there was a legitimate election to defer that much, then I think you have a legit deferral and legit late deposit issue.  Amending the return to remove the deferral (and not making it up) creates a train wreck of amending the 5500 too, and in theory a failure to implement a deferral election, although in the case of a sole prop that issue is somewhat blurred.

Thank you, Bird. You've confirmed my thoughts.