Because the oxygen sensor is installed in the exhaust manifold or pipe, which contracts when cool, it may be very difficult to loosen when the engine is cold. To avoid damaging the sensor, especially if planning to reuse it, start and run the engine for a minute or two before shutting it off, taking care not to burn yourself during the procedure. The upstream sensor, located before the catalytic converter, is also known as the Air/Fuel Ratio Sensor (Bank 1, Sensor 1), while the downstream sensor, located after the catalytic converter, is referred to as the Heated Oxygen Sensor (Bank 1, Sensor 2). Begin by disconnecting the cable from the negative battery terminal. The upstream sensor can be replaced without raising the vehicle; locate it in the engine compartment, unplug the electrical connector, and unclip any interfering coolant hoses as necessary. Use a special oxygen sensor socket to unscrew the sensor. For the downstream sensor, raise the vehicle and support it on jackstands, then access the oxygen sensor harness and unplug the electrical connector. If there is insufficient space to unscrew the downstream sensor, the relevant section of the exhaust pipe may need to be removed, and applying penetrating lubricant to the bolts/threads can assist in their removal. The best tool for this task is a special slotted socket, particularly if the sensor is to be reused, so take care not to round off the hex fitting. Unscrew the sensor from the exhaust manifold or pipe, then apply anti-seize compound to the threads of the sensor to facilitate future removal. New sensors typically come pre-coated, but if reusing an old sensor, recoat the threads before installation and ensure it is tightened securely. For the downstream sensor, reinstall the exhaust pipe section and tighten the fasteners. Finally, reconnect the electrical connector of the pigtail lead to the main wiring harness, attach any necessary connector retaining clips, lower the vehicle if it was raised, and test drive to verify that no trouble codes have been set.
Posted by ToyotaPartsDeal Specialist 
