The P0420 code popped up on my coop a couple of weeks ago. If I clear it, within miles ,it comes right back. It seems that this points to a bad catayltic converter.

Before I invest in a new cat, I just want to make sure that that is what the problem is. Earlier this year, I had a P0138 code. I replaced the O2 sensors, and I did not see any codes thrown for a month.

Could the O2 sensors I installed be related to the P0420 error. I have seen comments about gas causing the problem but that seems unlikely.

If I need a new cat, do I just get a after market replacement cat welded in or should I get a new Pipe/cat assembly? will the problem go away. I don't need an emmission test until next summer so I have time to get this fixed.
.

 

Which brand of O2 sensor did you put in? The MINI is particular to the NTK/NGK sensor which is the OE supplier. Look for 25648.

Any leaks on the exhaust manifold / cat? That could also trigger the code.

 

Intake air leaks can cause this to happen, too!
Dirty fuel injectors can also do this!
Make sure to use NGK BKR6EIX spark plugs, as they work the best by far and allow a good spark under all conditions. (the lack of using good plugs are a common reason for this code)
If you have done any ECM tuning, the fuel ratios may cause faults from changing fuel mixture too rich, or lean. The ECM wants to maintain close to 14 to 1 air/fuel ratio. It is vital to use onlu high octane fuel. (premium)

 

Thanks guys. Comments are much appreciated. I didn't get to work on my coop this weekend. Hopefully next weekend.

I am using low end O2 sensors so that could be it.

Intake air leak. I am guessing that would be between the mass flwo sensor and the intake manifold.

I need to get back under the coop and inspect the exhaust more thouroughly. Last time I looked, ( I was trying to find the source of a metal on metal vibration. It wasn't heat shield and it seemed to be coming from the muffler area) I did not see any obvious exhaust leaks.

The P0420 is problematic as it could be anything. I would hate to have this turn into a money pit. If it was the cat, I would replace it but it doesn't seem to be a cure all.

 

Let us know how it goes and what you find. My son’s R53 was on an original O2 sensor at ~215+ miles when that sticky code popped up.

Checked everything and since no other issues, I helped my kid learn a little and had him change the sensor.

About 200 miles later, code cleared.

One other thing, you may want to do is monitor short term and long term fuel trims and make sure the O2 sensors are switching/sensing or slow when the engine is on. Good way to confirm if they are functioning before you replace them (again)

 

Ok. Got to log the O2 data on my Mini, it definitely looks like the cat is not performing as it should.Although the voltage of the post-cat O2 sensor is not a exact copy of the pre-cat sensor, it seems to be moving around way to much.
I was thinking of getting a complete catalytic converter and pipe assembly. I think my cooper S only has one cat but a look online seems to show 1 or 2 cats. Any idea which configuration is correct?