Hey i just installed a PDM O2 Sensor Extension this last weekend because i've been getting the P0420 code. After install i cleared code and it has been working since, until today. Well more like until an hour ago, i was driving down the highway and up came the CEL! Damn it, i thought that thing would have fixed it?! Now what are the chances that my O2 Sensor is bad and thats the reason i'm getting CEL?? Is there anything else i can try before cutting wires?

P.S. I have CAI and borla headers, thats it i swear.

Did you have the cel with stock header? You could try header wrap to keep in some heat.

You may just need a new o2-air/fuel sensor.

Might want to check the O2 sensor. I have a CAI and Borla's on my RS, and I've never seen a CEL...

Did you have the cel with stock header? You could try header wrap to keep in some heat.

I did not get it with the stock headers. It didn't come on right away after i put the new ones on, took a little while, little less than a week i'd say.


You may just need a new o2-air/fuel sensor.

Yeah? Where could i find that? Both on the car and store wise.

Might want to check the O2 sensor. I have a CAI and Borla's on my RS, and I've never seen a CEL...

Is there an easy way to check it?

P.S. I just read about how having bosch sparkplugs can cause problems, and i think those are the ones i have. Should i buy new sparkplugs??

If you are using OEM sparks still, I'm pretty sure NGK makes those, and not Bosch.

P0420 actually stands for Catalyst Inefficiency, and is not usually thrown because of an O2 sensor, but rather a cat problem. I don't know much more about it than that.

Shot in the dark: If you used the stock gaskets when headers were installed, there could be an exhaust leak (doesn't usually throw a light though...)

It won't be the air/fuel aka front O2 sensor unless you are throwing CEL 0133. The 0420 is the cat inefficiency code thrown by the lambda aka rear O2 sensor. Some car's are just pickyer than others. My Cobb headers throw 0420's every 500 miles even with the PDM extension. Once I started getting the 0133 and had fried my front O2 sensor, I just gave up on it. I have smog coming up, so changed the front sensor and put the stock headers back on. No CEL for over 500 miles, so I know my cams aren't the culprit. Unfortunately, it is just something we have to live with when modding cars...

Yeah, with borla headers my o2 extender didn't work for me either. Running those borlas on some of our cars, the cel is just going to be there.

yea, i have the borla header, and stromug cat back exhasut, and a cobb intake, and have had no Cels ever. and ia have had them on for months.
Shane

i too have CAI and Borla headers.. no CEL

the ONLY time i got that CEL, indeed, my rear 02 sensor was bad.

i'd suspect the sensor iteself at this point.
you may want to inspect it for visible damage (my wiring was fray'd)

good luck
Jamie
www.subiegal.com

So i've gathered that the first thing to try and replace is the O2 sensor before replacing the CAT. Any chance that maybe replacing the sparkplugs with NGK?

I've had the cel since o2 I think. The dealer changed the cat twice and the o2 sensor once. After my warranty was up I just left it. Had the cel ever since. p0420. Maybe I should buy a new rear o2. They are pretty pricey though, is there an alternative? Anybody have a part # for a bosch or something?

Well in my case the extender didn't eliminate the problem, but made it easier to live with.

Personal Observation:
P0420 pops up more frequently with more highway driving, hence giving credence to the hypothesis that the borla headers cool down too quickly. Driving around town less air gets moved under the car and in the engine bay so the computer is happier. Those who have wrapped their borlas (Subie gal included) seem to do much better than those of us that like our borlas naked. I am pretty close to just wrapping mine. Lots of threads on this over at RS25.com.

i'd say change the rear 02 sensor, and if you get the urge, wrap the headers...it's not a bad idea to do it, but probably not nessicery....

My headers are jet-hot coated, still gives a CEL with the extension.