Help bmw x3 o2 sensors static voltage at 4v

[Marshalexese]

My car has been showing Chech engine light. I took it to local mechanic and scanned, oxygen sensors were the culprit.I bought 4 brand new O2 sensors after fixing them, the 2 upstream sensors show fluctuating voltage between 0.2-0.9v.But for the two downstream O2 sensors, the voltage is static at 4v with now variations. The voltage graph is also flat with no undulations.

Please I need help as this is affecting my fuel usage.

[MauiM3Mania]

Moved to the model specific discussion area.