Since the oxygen sensor is fitted in the exhaust pipe, which is contracting when cold, it might be hard to loosen when the engine is cold. To prevent breaking an oxygen sensor or damaging its threads, ensure that you turn on and run the engine a minute or two then turn the engine off without burning yourself in the process. Particular care should be taken when servicing an oxygen sensor because they can never be taken apart by a permanent pigtail and have an electrical connector which cannot be removed; they would be permanently damaged. Also, do not allow grease, dirt and any other contaminants to touch the electrical connector and the louvered end of the sensor and never use any cleaning solvents on the connector. Oxygen sensors are very sensitive and therefore they should not be dropped or mishandled and the silicone boot should be fitted properly to avoid melting. Removing an upstream oxygen sensor: Find the oxygen sensor at the upper end of the catalytic converter or at the end of the exhaust manifold of 2.5L engines, and locate the wiring harness and strip the electrical connector of the sensor harness disengaging it. Unscrew sensor with an oxygen sensor socket to use on tight areas, and in case of re-insertion of the old sensor, apply anti-seize to the threads, which new sensors are already coated. In the case of downstream sensors, place the sensor in the centre and following the catalyst, disconnect the electrical connector, unscrew the sensor and place anti-seize compound in the case of replacing the old sensor. Installation is the opposite of removal and it is imperative to ensure that the oxygen sensor is firmly screwed to the required torque.