Despite popular belief that the secondary O2 sensors are for emissions puproses only, I am seeing evidence that they actually have a fairly large impact on performance.
It all started when I gutted my pre-cats. I installed double-stacked spark plug defoulers to avoid the dreaded CEL from catalyst efficiency codes. When I did this, I did not drill them out at all as some have suggested. I had believed this was done solely to accommodate the sensor and instead used the spacers provided with the defoulers. After a short while, I began experiencing an significant loss of power during part throttle (almost no off-boost power) and my CEL began coming on every 800 miles or so. A quick scan with VAG-Com displayed P1176.
1 17584/P1176/004470 - Bank1: O2 (Lambda) Correction Behind Catalyst: Control Limit Reached
I did not think much of it at the time, so I just cleared it when it came up and continued on. At some point I decided to check the code in the Bentley and was very suprised by what I found.
Oxygen sensor control behind the catalytic converter is responsible for final control correction. It is the primary sensor and can override the signal from the oxygen sensor control before the catalytic converter.
Oxygen sensor control behind catalytic converter is superior to oxygen sensor control before catalytic converter and is the primary control. It corrects slight changes in the mixture (i.e. enrichment or thinning) by holding oxygen sensor control before catalytic converter at its highest or lowest point for a specific time (duration). If this time is in the positive range (i.e. 50 ms), mixture is shifted in the -enrich- direction. If this time is in the negative range (i.e. 50 ms), mixture is shifted in the -lean- direction.
After checking my fuel trims and seeing that they were all within a few percent, I once again disregarded it.
A few months ago I threw a secondary O2 Sensor heater code which activated my CEL. This prompted me to code out my secondary O2's. Recently, my CEL came back on, primary O2 sensor this time. So I did what any responsible Audi owner would do and replaced both primary sensors immediately. I did this thinking that it would solve all of my problems. It didn't.
Approximately three hundred miles later, remembering my suspicious P1176 research, I unplugged my secondary O2's as a last effort. Almost immediately, my part throttle power improved immensely.
I was searching through the Funktionsrahmen keyword "Hinter Kat" and had some promising hits, however, I recentely moved and don't currently have the internet on my laptop.
Could this have just been a result of adaptation slowly taking effect after replacing my primary O2 sensors?
Has anyone found any maps relating to secondary O2 correction?
Has anyone come across anything related to post cat correction in the Funktionsrahmen?
