Check engine light

There is a what's called" pinpoint testing" specifically for the code you are getting, whatever it is, you didn't post it. It's a group of tests to determine the cause and solution to your problem. However I'll give you a secret tip, not in any book, concerning certain Ford models and hego sensors, don't tell anyone else what I tell you, there is a separate ground wire specifically, gee I used that word twice so far, for the hego sensor, usually, but not always, orange in color. This wire has one purpose in life only, to ground the hego sensor. It is attached, on a few models, to the front of the engine block, but not always. What happens is corrosion builds on the ground connection causing the fault code, then people replace the sensor, which will not fix the problem. Also this ground issue can cause an intermittent check engine light, sometimes the connection works sometimes it doesn't. Solution is to clean the connection itself, possibly replacing it which can be done with basic electrical pieces. Remember this may NOT be the root cause of your problem, but may be worth the look anyway. Btw, how could your mechanic make the mixture richer? It's not possible. Also if he's using a scan tool, he should be able to erase the codes easily with the scan tool, not interfering with any components that may require a learn-to session, such as trans shift points and idling concerns.