Couple Check Engine Codes
Okay, so i've had a check engine light on for a while, and finally got it scanned.
The codes that came back were
P0420 - catalyst control system below threshold (bank 1).
I have a 2005 cobalt, 2.2 litre. What are the chances it's actually the catalytic convertor, and not the downstream o2 sensor? The o2 sensor is $50 for a delphi one, and the convertor is $400. I'm debating on spending the $50 for the o2 sensor just to see if that's the problem, but if it's just gonna be another $50 wasted (I was told by autozone that it was the upstream o2 sensor throwing that code), than I might as well just spend the $400 and be done.
Also, another code that was thrown, I can't remember the number, but it described it as this:
Evaporative Emission Control System Leak Detected (Large) - Occassionally, when driving, the car will ding and the 'check gas cap' light will come on, even though the cap is down as tight as it will go. Would a 'large' be classified as a faulty gas cap gasket, or is it something else somewhere.