"The issue with the CEL confusion and the 4 O2 bungs is this: the OBDII section of the DME expects to see different O2 levels pre and post cat to ensure catalytic efficiency, so having 4 O2 bungs in a non catted header is sure to get you a CEL. What you can do, is keep the factory secondary cats, or get high flow catted midpipes, and install a bung AFTER those cats and extend the wiring for the post O2 sensors. There will be a slight difference in resistance due to the added wiring but it SHOULD be ok. "
I know different folks have had good luck with extending the wires to the oxygen sensors and locating them after the downstream cats without getting CEL-issues. But in my experience this can still give CEL's. I have spent the last 6 months on and off messing around with trying to get rid of the CEL. Car is running great but when I added the German TTP headers and 100-cell cats, I started getting the CEL.
In an attempt to resolve the issue, we tried replacing cats, moving sensor bungs, moving sensors into and out of the exhaust stream, replacing sensors, we even stuck in an new "used" ECU, it has been quite fussy.
"I believe there are also some software reflashes that remove the catalytic efficiency monitoring, but I am not sure. "
As I understand it, shops that do that mod can get in big trouble, at least that is what the folks that wrote the software for the chip in my car said.
In my car the CEL would come on after 70 miles or so of driving. To try to get at the cause of the problem, I left my car at the shop and the fellow that wrote the software for my chip would drive the car with the Porsche tester plugged in and wait for the CEL to trigger and then note all the values.
After
several hundred miles of driving in this fashion, the "final" fix involved him reprogramming the ECU to have a modestly elevated threshold for the oxygen sensor level that triggers the CEL (as opposed to removing the cat efficiency monitoring).
As part of his trouble-shooting the problem, he ended up really tweeking and dialing in the chip for my car, the butt-dyno result so far is that the car runs and sounds better than ever, I cannt wait to get on a dyno to see what this latest ECU tune did.
So, this is one guys experience that messing around with the stock cat efficiency or extending oxygen sensor wires can result in CEL issues. This has been an expensive education.... have fun,
Ed
