I'd hate to bash your mechanic that you didn't need an O2 sensor, there is always a chance that the sensor he sold you is now bad too, depending on the brand he sold you. Honda's can be finicky about using Denso brand O2 sensors (I learned the hard way). That being said, we'd really need to know exactly what Trouble Codes you have in order to give any advice.I purchased it from Auto Nation. I live in Texas in a tiny town, so the closest dealer is 2 hours away. I took the car to a local mechanic shop who told me the O2 sensor. I have no idea about what DTC means. I literally just have to trust what I am being told. I dont have a clue about mechanics
If you haven't already, you may want to set up a Honda website name and password, enter your VIN # to find the service history for your vehicle as to what has been done. There are several Technical Service Bulletins (TSB) that may need to be performed. Taking it to the dealership is your best option. Hopefully they will cover some cost.