I have a 200 cell high flow cat on my 421 header. I have an angled O2 extender on it. Was at the VIR IMSA race yesterday and on the way back my Ultragauge started telling me there was a potential 0420 issue that later turned into a CEL. The nice thing about the 421 is that there is a ball/glove fitting that some call a doughnut fitting so the cat can be disconnected from the header pretty easy. I put the car up on stands and removed the Cat to make sure I did not have issues inside of it. When I got it off I noticed that the O2 extender was just a little loose. Figured between not being tight and the rain on the road, the rear O2 sensor was not seeing the proper heat. I took the extender off and removed the sensor. Heated the tip just to the point of getting some color and then quenched it in some water to shock any carbon. Put everything back together, cleared the code and took it for a spin. Code did not come back.

Got thinking on the matter and back years ago you put a small ball or chunk of steel wool in the extender. The tip of the sensor contacts this mass that in turn heats up more consistently. The sensor likes the steady heat better. Anybody done this with a MINI? There are "minicat" extenders out there and from I read some think they work and some think they are no better than snake oil. My extender is brass so I have eliminated the rusting that the cheaper extenders suffer.

I have a used OEM cat that I bought off ebay that I will put on prior to my car inspection. On high flow cats, I have some posts at the end of the below thread that has some basic engineering rules of thumb. I would like to find somebody that has tried a Yonaka 100 cell unit that has a S-wrap core.

https://www.northamericanmotoring.co...-please-4.html