Oxygen Sensor: Testing and Inspection

Fig 6 Heated Oxygen Sensors:

The upstream O2S sensor is located on the inlet end of the catalytic converter.

The downstream O2S sensor is located on the outlet end of the catalytic converter.

Each O2S heating element can be tested with an ohmmeter as follows:
Disconnect the O2S sensor connector. Connect the ohmmeter test leads across the white wire terminals of the sensor connector. Resistance should be between 4 and 7 ohms. Replace the sensor if the ohmmeter displays an infinity (open) reading.