P0138 Fault Code on 2000 Nissan Maxima: Missing O2 Sensor at B2 Position Behind Catalytic Converter

My 2000 Nissan Maxima is equipped with four oxygen (O2) sensors as per the factory specification: B1 and B2 at the front manifold, and B1 and B2 at the rear manifold. However, I can only locate three of these sensors in total. I have identified: - B1 Sensor 1 on the left manifold, - B2 Sensor 1 on the right rear manifold, - B1 Sensor 2 located at the front of the first catalytic converter. There is no sensor present at the back of the front catalytic converter (where B2 Sensor 2 should be) or on the rear catalytic converter. The check engine light activated, and a P0138 code was retrieved — indicating an issue with the B1/B2 sensor after the catalytic converter. I replaced the O2 sensor located at the front of the first catalytic converter (B1 Sensor 2), which caused the check engine light to turn off. However, this does not explain the absence of the B2 Sensor 2, which is supposed to be positioned behind the catalytic converter. The original design includes a B2 sensor after the cat, but I cannot locate it. Could anyone confirm if this is a known issue with the 2000 Maxima or provide insight into possible causes for the missing B2 Sensor 2 location? Any help would be greatly appreciated.