So a while back I got a CEL, pulled the code with my Scan Gauge, turns out to be O2 sensor. I put it off long enough, I was still getting pretty much the same gas mileage so I figured, whatever. Lately the mileage has plummeted. I routinely get 12 to 15L/100KM, I easily did 100KM less on my tank of gas. Screw that. I didn't want to go to the dealer, so I went to another garage that was supposed to be reasonable. Says that it will be around $175 and that it's mostly the parts that cost so much. Two seconds on Google showed me an O2 sensor for 04 Aveo is around $30. So wtf, I know often times you can find cheaper (cheaper quality too) online, but really?? I should have checked first, but at this point they have already ordered the part and I'm supposed to go back tomorrow to get it installed. I was told there are more than one O2 sensor (I already suspected as much though). How do I know the difference between various O2 sensors and how much they should cost?