So, my wife's check engine light came on 3 weeks ago. I scanned her car, expecting to turn up more of the same.

There was another code, P0301, cylinder 1 misfire. Her car had 145k miles on the original spark plugs, so I figured that was it. I replaced them. Her car ran better afterwards, so I figured it just needed a tune-up. A couple days later, she started it up first thing in the morning and it started running rough again. The check engine light came on as well. I told her to take my car while I looked more into what's going on. I scanned the car, and it threw P0171, system too lean, in addition to the ongoing cylinder 1 misfire. 

I was kinda puzzled. I didn't do anything with the vacuum lines while I was under the hood, but I suppose one could have fallen off while I was replacing spark plugs. 

I had ChatGPT open, so I posed the question, knowing that I should take it with a grain of salt. ChatGPT had me go through a couple steps, then it told me to push the throttle to 2500 RPMs and monitor long-term fuel trim. It went from +15.5% to +10%. I knew something wasn't right, right away. Engines almost always run much closer to stoichiometric when you push on the gas than that. ChatGPT said too much carbon has built up in her engine. 

Thinking about it, she had never carbon cleaned her engine before. I took it to the dealer, because nobody else around here does dedicated carbon cleaning. The dealer carbon cleaned it for $200. It's been a week and no misfire or lean codes are coming up anymore. 

I won't buy cars that rely solely on Gasoline Direct Injection, because of the curveball that threw at me. You basically need the dealer or a pro who specializes in carbon cleaning to take those cars to.