Check Engine Light On: P0420

[TH3B34ST]

I am not sure why I have not posted this sooner. My protege has a check engine light on and has been on for several months now. The code is p0420. Do you think that the cold air intake would be causing this? Is there a way to fix this? And one more question: The 4-1 header that i have on my car does not have a catalytic converter, so will I fail emissions test? There is still one converter underneath the car.

[vw.fe2o3]

Just taking a wag at it, but I don't think the cold air intake would cause that code. I think the lack of cat in the header is more likely the source. Read the link below on p0420 and you'll see the source of the code can be when the o2 sensors before and after the cat (that is supposed to be in the header) are reading the same data. If you have no cat, it stands to reason the sensors are reading the same unchanged data. Therefore the ECU thinks the cat isn't working properly.... and it would be right if there's no cat cleaning up the exhaust. Hopefully some others can chime in with some ideas.
http://www.obd-codes.com/p0420

[TH3B34ST]

I have until September to figure something out. (inspection) I really don't want to have to spend more money and find a different header that has a catolytic converter. I want to keep my 4-1 header on. anyone have advice?

[irishkev90]

What about moving the downstream (2nd) O2 sensor to a spot in the pipe after the second converter under the engine? Cut a hole, weld a bug and plug the old hole. Voila?

[crazycanadian]

P0420 is a code for your pre cat that would have been in the stock manafold... If you removed the stock manafold and didn't do anything with the rear O2 sensor this is why your code is setting... You'll have to make sure you have a good working cat thats down stream of your header and move the rear O2 sensor behind that cat.. Or else you'll have to reinstall your factory manafold/cat set up...

STAY AWAY from high flow cats.. They are garbage and will set P0420 codes! Walker CalCat are the only ones I know of that are OBD2 compliant and wont set P0420 codes...

[TH3B34ST]

I took my car to a monro muffler and had them look at it. The guy said that he could custom install a catalytic converter, which would allow me to keep my 4-1 header on. I can either do that or buy the stock header+catalytic converter. Both cost around $400. What do y'all think of these options?

[irishkev90]

Keep the 4-1 I'd say. If it costs just as much as going back to stock, I see no point in going back. I personally would prefer a 4-2-1, but that's just me