I own a 2005 mini cooper s that has had cel since I have owned it, every time I pull the code it says that the rear o2 sensor is reading lean. I just did a major service to the car, changed the supercharger oil ,added an overdriven crank pulley, changed the oil and coolant, and in this process I replaced the rear o2 sensor with a brand new one. I reset the code but it came back on and is the same one as before. Can someone please help me figure out what's going on with it? Note. it does not come back on while I am driving. I just reset and went for an almost 10 minute drive and took it almost all the way to redline and no code, but as soon as I got home and shut it off, I turned it back on just to see and instantly the code was back on.

 

Sounds like you may have an exhaust leak in your exhaust manifold between the two sensors. The stock manifolds are notorious for splitting a weld at the joint before the catalyst.

 

I will deffinatly check that out, thank you!

 

Here is a post you can reference for the possible leak.

https://www.northamericanmotoring.co...-manifold.html

 

So I looked into that thread you posted a link too and I was curious if you knew if deleating that pre-cat would cause a check engine light due to the downstream o2 sensor reading rich? P.S I live where there are no emissions.

 

Yes it will. You can put a O2 eliminator elbow on the rear bung and that usually fools the rear sensor enough for the computer to not throw a code.