There's a wire from the wheel sensor, that may have been dislodged during the install.
The man who ran your codes may possibly be looking at your car as a steady moneymaker.
Have you written the odometer mileage on all your gas receipts? That will tell you your real gas mileage.
BTW, my wife consistently gets 15% better mileage than I do. So your mpg issue might be that right foot. ;+}
How much did he charge you to run your codes? I was just at Oreilly, and they had a code reader for $40. Cheap POS, but useful.
If it really WAS your oxygen sensor, there's a dash light for that. And a code for that.
It may be about time to replace it anyway, but no light and no code means no problem.
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