When fixing an oxygen sensor, then be careful because it is located at the exhaust system and when is cold, then steel contracts and becomes difficult to loosen. Do not touch the thread where the sensor is fixed, or the sensor itself, as this may cause harm; allow the engine to run for one or two minutes then turned off. The oxygen sensor comes with a permanently attached pigtail and electrical connector that should not be disconnected because their damage will compromise the sensor's function. Exclude grease, dirt, and other contaminants from the electrical connector area and the louvered end of the sensor; no cleaning solvents should be used on it. Do not drop or be harsh on the sensor especially when handling the silver looking sticky part This is because the having the correct positioning of the green silicon boot at its correct place is very important not to melt and facilitate its functioning. To access upstream sensors: Open the engine and exhaust manifold cover, disconnect the electrical connector, disconnect the sensor wiring harness from the clips, and if possible use an oxygen sensor socket to loose the sensor, if you are able to you raise the car on jackstands to have better access. An anti-seize compound must be applied on the threads if the older sensor is reinstalled on the system; new sensors will automatically have it applied there. Installation proceeds in the opposite direction, and one can tighten the sensor firmly. For downstream sensors, secure the car on jackstands, isolate the wiring harness from the sensor's pigtail, unscrew the sensor, if possible, with an oxygen sensor socket and also apply anti-seize compound if reusing the old sensor. Once more, the process of installation is the reversal of the process of removal, followed by ensurement of tightness of the sensor.