For the 2001 maxima there is a bulletin for a reprogram for the 02 sensors after replacement to update the control unit. the # XXXXX (NTB02-002) for the bulletin repair.
I think this will fix your problem, I hope this helps, ROY!
The reprogram comes from nissan, only the dealer has this ECM reprogram for your Control unit. they have to down load it into there scan tool and install it into your car.
Thanks again! ROY