It was first a fuel sensor &/or an oxygen sensor or both. I had the same issue twice within a few weeks of repairs. They saw white stuff on the sensor & sent it to corporate headquarters. The fuel was being flooded too much for the settings. They had to reset everything twice. So far so good, but it was a weird situation for a while, especially since no one had a clue why this happened.

Update from Nov 30, 2019: Oh, the fuel mileage was down to 12 miles a gallon when this problem was occurring, but that is not saying much - since I only average around 16-18 miles a gallon.

My check engine light turned on a day after filling up my gas tank. The gas cap says to turn until you hear 1 click so that's what I did. After going to the dealership, they said it's a very common issue with the plug-in hybrid and many people are coming into the dealership service with the check engine turning on due to the gas cap not being tightly closed. This causes gasoline fumes to leak out to the outside, which triggers an EVAP (evaporation) error. This causes no harm to the engine but it does harm the environment. The design of the gas cap is bad and owners need to make sure to keep turning and tighten the gas cap. Even though it says on the gas cap itself to turn until you hear 1 click, you have to keep turning until it's tight! The dealership service plugged in the error code reader to confirm that this was the issue and simply reset the code to make the check engine light turn off.

This is actually the second time this has happened to me. The first time it happened was a week after buying the car and the dealership filled up the gas tank for me and detailed my car. So even some dealership workers are having trouble closing the gas cap on this car.