Low voltage is the failure mode of the O2 sensor. It causes the ECU to add more fuel than necessary, rather than dangerous lean condition.
A false low signal can also be caused at idle or coast down by a vacuum leak somewhere between the air sensor, and the head. This is unmetered air entering the engine.
Common causes of that are bad EGR gasket, bad intake gasket, or bad gasket on idle air bypass valve.
John C.