Diagnosing and fixing cars wasn't really the thing I enjoyed about this game, even though that was oddly satisfying, and the match 3 mini-game was a nice touch. (And of course, watching your money increase is a lizard brain thing.)
What I enjoyed was swiping parts from my friends when they placed an order for their own parts. Though that didn't start happening til later in the game, after we used up the parts sitting around in the workshop.