After playing a new game for a little bit, I moved my line to make things a bit more efficient. Apparently, I took it too far and made the line so efficient, that engines, radiators, and exhaust are no longer necessary components required to sell vehicles.
The picture shows how production goes from dry finish to brake assembly. There is also no imports for engines, radiators, or exhausts.
The game is still playable and no errors have been popping up, I just happened to notice I was doing very well and not spending as much as I should have been per car. Has anyone else experienced something like this?