Description of operation: Burn-off
WARNING: This page is about a different car, the 2001 Saab 9-3. However, it is still accessible from the selected car via links, so may be relevant.
To maintain the best possible contact in the sensor connections, TWICE sends a voltage across pins 5 and 21 to burn off any oxide. This is done for 1 second with the ignition ON.
To ensure that the burn-off current remains sufficiently high, a diode is connected in parallel across the sensor connectors. The diode conducts at low resistance, which means that the burn off current passes through the diode instead of the sensor.