Hello! I'll try to be as detailed yet concise as possible. To start off, I did work as a mechanic several years ago before switching careers. I was by no means a master tech, but I have a pretty strong grasp on how cars work and how to repair them. That being said, drive by wire/the amount of electrical components controlling the engines is a bit out of my wheelhouse so please be understanding if I'm not super knowledgeable!

Okay, so I have a 2015 Mirage, about 85k on it. The car is not throwing any codes. Never had an issue, kept up on oil changes, etc. Over the last 6 or so months, the car would idle a bit rough at times, and maybe stall out once or twice. I'd hold the pedal down to trigger clear flood mode and all would be good. Flash forward to two nights ago. I'm driving home from work and every time I get to a stop the car stalls. It wouldn't start unless I initiate clear flood mode. Okay, clearly running rich. No worries, I figure I'll park it and see what happens tomorrow. Well, yesterday the car doesn't start. I google and see we have a TSB on these cars for this reason. I call a dealer and they confirm my car does need an ECU reflash. Wonderful! I get it towed down there. I get a call saying the reflash is done. I ask if it's running and the service writer says no, it needs spark plugs. Okay, makes sense, I guess they could have been so fouled from the rich condition. I thank him, and tell him I'll take care of it. I go to the dealer, throw some plugs in, and... no start. I go in and ask if his tech was absolutely sure it only needed plugs. He explains to me that no, that was just a starting point, but that the plugs were so fouled we had to start there to get it running. I asked him if the reflash would have helped that, and he explains to me that the reflash I got was for the SRS system. When I tried explaining to him that the 2015s have an issue where they run rich, and show him the TSB, he says it doesn't come up for my car, and shows me in the system that it doesn't show up as needing it. He then tells me that my ECU might be fried, it could be more trouble than it's worth, etc.

I left the car and am awaiting a call back, but I'm not super hopeful this dealership knows what's going on. All that being said my questions are:

1.) Are ALL 2015 Mirage's plagued by running rich? Or is their validity that it wouldn't show up if it didn't need it?
2.) Anything else I should check?

Thanks guys!