...they did to repair the issue each time? Was it as simple as an adjustment of a sensor, or did they actually replace something?

I had a Oldsmobile back in the 80s that needed a new carb, but three different GM dealerships didn't want to do that, so they made an adjustment to the carb that lasted about two weeks before it would go full rich and foul the plugs. That was a good lesson to never trust any GM service department. They will do the easy & quick repairs while you're under warranty, but won't actually correct the problem until your warranty runs out.