If you can time it right.....get down to autozone with the CEL on and they can use a diagnostic tool to get a code. And best part is that it is free. I had both o2 sesors damaged because of the shitty shop that installed my header(MIDAS). The car really doesn't seem to run much different except for gas milage and possibly running rich. Get the code then you can tell which O2 sensor it is. There is one pre-cat and one post-cat. Also....go with Sohfast.com or HAP recycling to get an O2 sensor. Alot cheaper that way. O2 sensors don't go bad very often or quickly unless physically damaged.