Originally Posted by ZugyNA
Well....if the second O2 has the final say...it's possible that using the EFIE on it would work? Using it there would tell the ECU that the mix was too rich and over ride the first O2?
I think that having an EFIE on both 02 sensors would be best. In terms of dominance, everything I have read about "closed loop" operation implies (to me) that the 02 sensor in the exhaust manifold is the one that is primarily used by the ECU/PCM. The second 02 sensor (again, from what I have read), is helping to maintain the health and efficiency of the catalytic converter.
Hypothesis : At least one 02 sensor is needed for closed loop operation.
Test #1 : Disconnect first 02 sensor and see if car is able to go into closed loop operation. Note OBDII error codes. Reconnect sensor. Clear error codes.
Test #2 : Disconnect second 02 sensor and see if car is able to go into closed loop operation. Note OBDII error codes. Reconnect sensor. Clear error codes.
For each test, a failure to enter closed loop implies that the ECU/PCM depends on that sensor for closed-loop operation. If both tests fail to enter closed loop, then both sensors are absolutely necessary for closed loop operation.
Here is another thread by XFi on the same subject :
Just ordered an EFIE - BrightGreen FeverBuster
I think that someone around here (XFi?!??!) was going to install an EFIE on both 02 sensors, but I can't remember who.