Lexus IS250 is getting code P0051, needing oxygen sensor replaced

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Lexus IS250 is getting code P0051, needing oxygen sensor replaced in Bank 2 Sensor 1 (located before the catalytic convertor on driver side). It has all the symptoms and is 91,500 miles, never having had them replaced before. A mechanic told me they'd have to take out the manifold to access the sensor, which would include replacing the gasket. I saw online videos and there's no mention of having to remove the manifold to access the sensor. In fact, one guy accessed it on driver's side floor near gas pedal, instead of going underneath car. Who is correct?