4Pcs Up & Downstream 02 Oxygen O2 Sensor For Ford F-150 3.5L 2015-2016 Turbo

Downstream Left:FL3Z-9G444-C Downstream Right:FL3Z-9G444-C. Q :How do I know the O2 sensor is good or not ?. Q:Why does the oxygen sensor have false alarm, after installation ?. For Ford Parts. Code checker identifying O2 sensor failure.