Originally Posted by gsg740

I just got my Brek HFC installed last week, after 100mile it throws a P2A00 CEL. (A/F Sensor 1 Bank 1 Circuit range/performance)
I cleaned the code & learned fuel trim. But same CEL came back again after 2 days.
I cleaned MAF, checked intake leak and used soap water method to test the exhaust leak. But I didn’t find any leak at all. The car sound normal except some rattle/hiss under WOT. So, if there was an exhaust leak then it must be very small.

I took some logs, it shows the P2A00 code is caused by different A/F ratio between B1 and B2. My bank 1 is constantly richer than Bank 2 by 0.5-1. (Bank 1 is about 13 and bank 2 is around 14)
The interesting problem is that every time after I cleaned the learned fuel trim. Both banks produced the same A/F but after a while the ECU starts to add A/F adjustment on Bank 1, which makes Bank1 richer and richer. Eventually cause the CEL. The speed of ECU adding A/F adjustment is very quick, 100 mile after reset the A/F adjustment on bank 1 is 0.33 while bank 2 is only -0.002.
I used the same map as before so I don’t think the map will have any problem.

I have an additional Innovate A/F sensor in the drive side (bank 2) of HFC but not in passenger side. The Innovate’s sensor was put in the bung that was on the Brek HFC.

I’m not sure what cause the problem. But my investigation is narrow down to either the additional O2 sensor cause the problem or there was a small exhaust leak that I wasn’t able to find.
Now, My question is which is more likely to cause this type of problem? And is there anything else could cause this type of problem?

My car is 2011 nismo, It’s just under 15k