I left the gas cap off after filling last week, and when I got home my Mercedes had its check engine light on. Unfortunately it's due for inspection! I didn't want to spend a half hour of my mechanic's time to turn it off, which would have been around $50. For half that, I've resolved it myself and now have a neat diagnostic tool!
Basic auto maintenance
If the check engine light is on, one of the first things to check is the gas cap -- the light comes on when the cap is not on, or not on tight.
I drove the vehicle another hundred miles or so after this happened, and it still didn't turn off. With the end of the month approaching, I didn't really want to get pulled over. So I researched it, and found an OBD-2 device which communicates via Bluetooth. It was $20: https://www.amazon.com/gp/product/B005NLQAHS/ ; image from Amazon:
From the YouTube videos I had watched, I knew I'd need to spend another $10 on the phone app. I never buy phone apps, but this one was worth it. And, it turned out to be only $5, so beat my expectations.
Plug it in, turn the car on
The plug is "backwards" in my car, meaning when I have it plugged in, the lights on the device are facing the front of the car and I can't see them. I wanted to be sure I could see the lights before moving on to the next step, so I used my phone as a "mirror" (using the front-facing camera) and saw them.
It connected just fine, using the "free" version of the program ("Torque Lite"), and I could see the fault but not clear it. So then I spent the $5 on "Torque Pro".
Resource contention
Note that you should exit Torque Lite before attempting to connect Torque Pro! I had a momentary feeling of buyer's remorse, but then realized that I had left the Lite one connected; exiting it fixed the issue.
Some vehicles reset while running, others while not
I tried three times to reset it with the car running, as the YouTube video I had seen demonstrated. (However, while that video did show a Mercedes, the dash was different from mine -- mine has the words "CHECK ENGINE" whereas this had an image of an engine.) After it failed the third time, I looked at the instruction booklet in the "troubleshooting" section.
It had a section on resetting the check engine light, and the answer included that some vehicles must be running to reset it; and some vehicles must not be running. So, I turned the car off, put the key in "ACC" mode (almost turned on), and reset it and it worked!
Any car since 1996
I can now diagnose any car sold in the US since 1996. Not saying I'm going to start an auto mechanic's shop, but I am very happy with this outcome, as I didn't just hand over some cash for a result -- I spent half the cash, got the result, and can now achieve future results with this new tool and skill set!