Well if you are confident the sensors are operational then this could be one of three things:

a fueling issue
a situation where something else is getting into the cylinders (oil, like a seperator failure) and leaning that bank out
the emissions control/evap system has failed and that caused the lean reading.

I am not sure the last thing is true or possible, but hopefully someone with evap system experience can clarify that.

How many miles are on your car?

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2001 E53 3.0 5pd