Hey i just recently took my car to emissions and the check engine light was on so i knew it would fail but i took it anyways they hooked the scanner up and gave me a print out of the fault codes. P0420 Catalyst System Effeciency Below Threshold (bank1)
PO133 02 Sensor circuit slow response (bank1 sensor1)

I'm guessing that the 02 sensor is setting the P0420 code but i'm wondering which 02 sensor it is, i haven't looked yet to even see if there is more than one but i think all OBD2 systems have more than one. my guess is that it is the one up in the front right off the headers but i just want to clarify it.

Also have you all heard of the CarChip that acts like a scan tool? its really small and just plugs into your OBD2 port under the steering wheel. It will track speed and revs for a given amount of time and i think i heard that you can change air fuel ratio with it also?? i was wondering if this little thing is true it runs for about 120$ and comes with software to plug into your computer the website is www.davisnet.com could be a cool little toy

 

oops that probably would help its a 2000 impreza RS