I just bought my cobalt and it is throwing a code p0420 which is something with bank 1 catalytic efficiency (whichever o2 sensor that is).

From what I can tell from above one o2 sensor goes into the exhaust manifold and one goes into the stock dp(correct me if I'm wrong) , what I need to know is which o2 sensor is the problem one with that code?

 

420 should be the one after the CAT in the downpipe, I think 137 is the one in the exhaust manifold.

 

Awesome. Thank you!

Any suggestions on brand of o2 sensor, or if there is a difference in brand at all?

 

I'd say probably not much difference in brand, but personally I go with ACDelco parts when I can. I was going to link you to rock auto to buy the part (just ordered both o2 sensors and both MAP sensors for my car from them and it's hard to beat their prices), but it appears they don't sell stuff for lsj's so I'm not sure where you want to look next.

 

Chances are that it's not the 02 but the cat is the problem, the p0420 code is common with aftermarket down pipes. I say throw a 90degree o2 spacer and calling a day cheaper than both an o2 and cat

 

I started a thread in the fuel economy section about this because it seemed a little more fitting once I realized how bad of gas mileage I'm getting along with the code. I don't have an aftermarket cat or anything is what's confusing about it