Hey guys...I love this forum in that helps me with my car. I have a 2003 spa yellow s2000 with 68,500 miles on it. I bought it six months ago and a week after i got it, I got a cel and the mechanic told me to change the cat. I changed the cat and now 6 months later, I get a p0420 cel code. The guys at autozone tell me that i need an o2 sensor. And i need to know which o2 sensor cause theres a front and back one. If i do, can i do it myself. Please help guys, im a student and I need this car perfect. Thanks alot.

 

If O2 sensor is the cause, then it's the secondary one, the catalytic converter.

Jason

 

Hey jason, thanks for the reply...so which one is the secondary one...cause when i see o2 sensors, i see before and after converter...and does anyone know if u put anti-seize grease on the area that the o2 sensor screws in the cat?

 

it is the one that goes into the cat.

 

So i just ordered the secondary o2 sensor...I drove a while to eat and the light was still on...after i finish eating, theres no check engine light!! Is this right? Should i still put it in, or just keep it until it happens again, or try to return it...

 

Ok WARNING.

First, the CEL for the secondary O2 sensor is meaningless. It doesn't affect anything, you just won't pass emissions testing.

Second, if the 2ndary O2 is shot then the problem isn't a bad sensor (yes, it's broken...) it's a bad cat. The core of the cat has come loose and sheered off the tip of the sensor. If you put a new sensor in there then it too will sheer off the next time you go over a set of railroad tracks and you'll be out $300 for nothing.

The way to know is to pull the O2 sensor from the cat and look at it. If it's broken your cat is broken and you need to replace it BEFORE installing a new sensor. Just put the sensor back in the cat and keep your eyes open for a used cat. People sell them cheap when they buy test pipes. Keep that new O2 sensor for after you replace the cat.

 

I have just changed the cat about 6 months ago...i saw when the o2 sensor was out...it was not broken...it was in one piece and looked ok...only problem is that the CEL is now off...and i have already ordered the o2 sensor...

 

Isn't there a code to tell you if the o2 sensor is bad? Then again, who knows if shearing the head off will trip that. P0420 is the code for "catalytic system below efficiency (bank 1)" A lot of us MY06+ guys with test pipes know that one quite well