The ECU will only look at the secondary O2 sensor during closed loop operation (i.e. lean burn, steady freeway cruise, idle, etc). The ECU will make small adjustments to the fuel trims based on the small adjustments made during closed loop operation. This is on the order of a few percentage points. I don't recall ever seeing anything over +/-5% in the last couple of years.
If you guys have ever called Berk for tech support you've heard us say, "Clear the ECU learned values before running the HFC's. You will run less of a chance of throwing a CEL light". Over time your ECU may have adjusted based on your driving habits. By clearing the fuel trims you give your ECU a chance to start back at zero and readjust to the new cats. The cat is in essence an oxygen storage device. When installing a BERK HFC that flows twice as much as the OEM cat it does not have enough contact time with the catalyst to "scrub" the exhaust gas and let the reaction occur. So the O2 readings will be different. Most of the time the thresholds of CEL or no CEL are large enough that the ECU will readapt and the CEL will stay off. However some of you who were already operating on the outer range will pass that threshold after a few drive cycles and illuminate the CEL light.
During driving situations where the engine sees load it will completely ignore the secondary O2 sensor and go off of it's pre-programs maps. On one of our race cars we actually did not use any O2 sensors at all! We would run completely off the pre-programed maps on the stand alone. We would make small tweaks to the maps for high altitude race tracks and summer/winter weather.
So, the moral of the story..... the secondary O2 sensors technically DO make tiny fuel adjustments in concert with the primary wide band sensor in the header, but only to make the car run as clean and lean as possible during steady cruise. From the stand point of the car enthusiast, they don't do squat!
And to set the record straight in plain black and white:
Berk HFC's are not SMOG legal in CA. Can they pass a sniffer test? Yes, all the time. But Berk Technology does not guarantee emmisions performance.
The vast majority of you will never thorw a CEL. However there is a very small percentage of you who will throw the occasional P0420/P0430 CEL. Down the road there will also be a very small percentage of you who throw those CEL codes with the OEM cats!
Hope that gives you some good info to chew on!
Cheers guys!
Berk Technology