Okay When I was trying to pass smog, I replaced my oxygen sensor with a new Supposedly AC Delco Oxygen Sensor. Now I did get it from a guy off eBay, but it looked legit...

I noticed on my next 2 tanks of gas my mileage went DOWN by about 30 miles per tank... I also noticed my idle was rougher in closed loop.

Now I didn't change my driving habbits, and I drove the same way, same amount of mileage on the freeway and around town. Basically, everything was the same as far as driving before and after. So I know that wasn't it.

I went back to my old o2 sensor, and my mileage returned to normal, and the car seems to feel better overall.

Could this sensor be a fake AC Delco, or could it have just been bad from the beginning. I can see it's new, but could it have been damaged from being dropped or something? How can I tell it's a real AC Delco, there's no markings on it... it does have a purple wire and the standard connector. It does have a tag with a number on it, the number is 25347711, I couldn't find any reference of that part number online...

What do you guys think?