2nd Gen R56 Cooper S Most liked posts in thread: P0420

I thought the 420 code was used strictly for the rear O2 to monitor the catalytic converter? I was always under the impression that if the pre cat O2 went bad, an A/F ratio system too lean/rich code (can't remember the numbers) would pop up if that sensor was going bad. Am I mistaken? I have a RMW street header and have been getting the P0420 code since the install and always thought it was because of the problems with the catalytic converters used during fabrication. I bought an O2 simulator to install in hopes of clearing the light so I can get my car inspected (its only overdue by a year!!)

Steve

 

Hmmm. I guess the 420 code means something different to everybody...:biggrin5:

 

Yep, I get em all the time. O2 sensor probably on its way out. Probably happens after a hard acceleration and quick lift, at least thats how mine does it. A few days of driving around like a sane person clears it for me or I can clear it at will.

 

Could this be caused by an exhaust leak, too?

 

Or a bad cat? Yep my Scangauge sure comes in handy

 

I would really like to know too. What is the group experience in resolving this code?
I currently have that code and an inspection coming up, those 02 sensors aren't cheap.

 

I'm currenty using back tape to good effect.

 

Are you still using the OEM header?

 

Yes I am....

 

No, I'm using an early 1st generation OBX header that has been on my MINI for years.
I would just like to know what others have found to be the cause of the not so rare
P0420 code.

 

A not so good seal at the header/cat-back joint can throw it. Only gasket i have found to be worth poop is the oem one. Had the code, replaced that gasket, and code no more.

Sent from my DROID BIONIC using Tapatalk

 

I have a RMW Header and Milltek Cat back and had the same code a while ago.

It was NOT the header but a crap gasket I used that caused the 0420.
while I was troubleshooting, I switched the O2 since they are the same front and back, cleared the codes and reset the adaptives and the code still came back.

I ended up replaceing the crap gasket (Vibrant 2-Bolt High Temperature) with a Remflex graphite exhaust gasket and this worked for me.



Remflex Exhaust Header / Manifold Gaskets

 

Good info so far, searched the net for PO420, all makes and models throw this code.
I'll bet across the board a few million has been spent by people trying to resolve this code. Maybe the EPA/DOT allowed this code as a bone for the Stealer/Dealers to corner and gouge us devotees to the internal combustion engine.

 

Most aftermarket high flow cats will throw this code.

 

Yep that's why my mounted Scangauge comes in very handy :cornut: And for my also works for my P0128 thermostat code in the summer

 

Nathan what device are you using to "clear it at will" ?

 

I'm using a bit of software called Dyno-Scan by Auterra that I bought years ago for my blackberry. Only reason I still keep that old phone.

 

I used two anti fouling spark plug bungs to keep the rear O2 sensor honest, works like a champ. Going to get the stainless steel bung now that I know this works.
Just Google it, "universal 02 simulator cel fix"

 

^^^ Some people been using these.....

O2 Simulator Mini Cat Cel Fix

 

I did it with the HELP parts from Pep Boys for about $6. bucks. Will spend $15. + shopping for the stainless steel one from big daddies though.

http://www.bigdaddiesgarage.com/straight-cel-fix.html