While working with oxygen sensor service care must be taken as it has the permanently attached pigtail and electrical connector that cannot be unplugged, and if they are damaged, the sensor is finished. To ensure the longevity and proper functionality of the electrical connector and sensor, grease, dirt, as well as other contaminants must not come into contact with both the electrical connector and the sensor; the sensor must also not be cleaned using cleaning solvents. Also, minimize either leaving the area around the sensor or dropping the sensor or handling it roughly. As the sensor is fitted in the exhaust manifold or 
Catalytic Converter where there is a contraction when the system is cold, it becomes a little tight to loosen when the engine is cold, then a brief running of the engine will help. The inlet and outlet of each primary catalytic converter is fitted with sensors together with two heated oxygen sensors at the outlet of each exhaust manifold before and after each catalytic converter. For the V6 models, the only way might be by lifting the car and setting it on jackstands to ease access particularly to the rear manifold sensors. To get to the sensor, eliminate all elements that can create interference and it differs depending on the year and equipment; sometimes preliminary oxygen sensors can be reached through openings in the heat shrouds. Removing the wiring, take off the sensor with an oxygen sensor socket or large wrench, and then cut the sensor pigtail off. Remove the sensor, and knowledgeable people would inform you that there are specific oxygen sensor sockets that can be bought at most auto part stores. Replacement is the process of putting back parts in reverse order but while fastening the oxygen sensor threads are covered with the anti-seize compound and tightened to the recommended torque.