Oxygen Sensor & O2S Control Before TWC, Check - Passat Sedan 1998
O2S reference voltage, checking
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Disconnect 4-pin connector for respective oxygen sensor.
Oxygen Sensor (O2S) behind Three Way Catalytic Converter (TWC) -G130-, green connector (arrow)
Oxygen Sensor (O2S) 2 behind Three Way Catalytic Converter (TWC) -G131-, brown connector (arrow)