Got approved for federal extension but went ahead and filed forgetting that I needed to contribute more to my solo 401k

[relativist]

I filed for a federal automatic extension by using an electronic payment method via the irs site, got approved.  But then went ahead and filed the federal tax form today and it was accepted.  But I forgot I was holding off on filing my taxes (hence why I got the extension) to make a contribution to my solo 401k, which I know I technically would have had until October 15 to contribute.  Even though I filed for an extension until October 15, but I already e-filed just now, can I still contribute to my solo 401k by October 15, or at least in the next few weeks without amending my return?

(My other question identical to this was closed without answers after only a few hours so posting again because that seems like it was a mistake)

[IreneS · Accepted solution]

UPDATED FOR TAX YEAR 2019

Yes, even though you already filed your return the actual contribution can be made up until the personal tax-filing deadline (April 15, or October 15 if an extension was filed).  NOTE:  The filing deadline has been extended until July 15, 1010.  This means you have until July 15 to make the contribution!

If you didn't include the contribution on your tax return and you want any tax benefit from it, you'll have to amend your return.

[Edited | 4/15/2020 | 1:45pm PDT]