Couple Check Engine Codes

[ex0r]

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.

[Evan]

I don't know about your first problem but for the second one I would try replacing the gas cap just to see. Get a nice locking one or something they're like 12 bucks I believe.

Also, stupid question but are you turning your gas cap two clicks to tighten? That's what I was always told to do...just saying.

[ex0r]

Originally Posted by Evan

I don't know about your first problem but for the second one I would try replacing the gas cap just to see. Get a nice locking one or something they're like 12 bucks I believe.

Also, stupid question but are you turning your gas cap two clicks to tighten? That's what I was always told to do...just saying.

I actually do three clicks. My wife seems to be the one having a problem with the gascap LOL go figure. I looked at the gasket on the cap and it looks cracked so I think I'm going to replace it and check again.

My big problem is the first code I'm getting.