[Gaz64]

Something to think about.

If your O2 sensors are only looking at one cylinder per bank, and that cylinder misfires intermittently, the O2 sensor reads lean (high oxygen from no combustion).

If the ECU retrims the fuel accordingly from a momentary lean condition, the whole engine will be trimmed in the rich direction.

A permanent miss in that cylinder will have the engine go rich as the ECU trims will allow.