Trying to visualise and understand how the ECU interacts with two sensors, and thereby making minute changes with the volume and maybe pressure of fuel delivery is mind blowing. The first time I removed a pair of O2 sensors I was shocked that the upstream sensor was clean, but the downstream sensor was black.I expected it to be the opposite.
The tailpipes on my S1 Roadster are always clean, but my GDI was sooty from day 1. Again, logically for me, a clean tailpipe means full combustion and low emissions, but that doesn't seem to be the case.
Using a K&N on my Roadster put the Lambda so weak at the MoT, it was on the edge of a fail, and apparently may also indicate an air leak in the manifold. But the standard filter fixed it.
In terms of the two sensors governing mixture, I imagine that they also talk to the MAF, and throttle position sensor, maybe also revs. How mixture converts into CO and HC emissions should be basic chemistry, but how that looks in the software map is anyones guess.
