If my 2017 self employment taxes need amending regarding a Section 179 asset deduction, can I make the change on my 2018 taxes before the amended 2017 forms are done?

Yes you can adjust the 2018 and mail the 2017 Amendment within a few days.

That will prevent two amended return.

Thanks!  In that case, I could  use advice for how to prepare my 2018 info correctly. The error I made for 2017 was entering a business vehicle that I purchased and put into service in 2017 as a Type G Asset, which is the category for 'General Machinery'. This gave me a Section 179 deduction for the vehicle, but it also caused the vehicle to not be correctly identified as a vehicle in the first place, and so TT is trying to give me a vehicle deduction this year even though I should not get one. So for 2018, should I delete the vehicle from the Assets section entirely, and enter it only under the Vehicles section?  And if so, should I flat-out delete it from the Assets section, or state that the Asset was taken out of service for 2018? (I would have thought that vehicles belong in the Assets section, but since there is no specific category type for them, it seems that is not the case...?)

Delete it from the Asset section and add it in the vehicle section

Thank you!

Although... since the full section 179 vehicle deduction can only be made in the same year the vehicle was purchased and put into service (correct?), that means that I may have to amend my 2018 no matter what since it will not be able to account for the fact that the cost of the vehicle should have been fully depreciated last year... ?