Remove the Oxygen Sensor
Start the engine and let it warm up for about three minutes. Then turn off the engine. This will help remove the oxygen sensor in case the sensor threads or mounting screws have seized onto the exhaust pipe.
Detach the ground battery cable using a wrench.
Locate the oxygen sensor you want to remove. Depending on your particular model, your Isuzu Trooper may have one or more of these types of oxygen sensors: an upstream sensor (located between the exhaust manifold and catalytic converter) and a downstream sensor (located on the back exhaust pipe).
Raise the front of your Trooper using a floor jack and support it on two jack stands, if necessary.
Unplug the oxygen sensor electrical connector. On some Isuzu Trooper models, it is necessary to remove the passenger seat and partially lift the carpeting to access the downstream-oxygen sensor connector.
Remove the two mounting nuts from the oxygen sensor using a ratchet and socket, if your particular vehicle model is equipped with the flange-type sensor. Otherwise, you may use a box-end wrench or oxygen sensor socket, if it is equipped with the screw-in type sensor.
Remove the oxygen sensor form the vehicle and discard the gasket if you are replacing a flange-type sensor.
Install the New Sensor
Apply a thin coat of electrically conductive anti-seize compound to the threads, if you are replacing a screw-in type sensor.
Start the two mounting nuts or the sensor by hand on the exhaust pipe to avoid damage to the threads. If you are installing a flange-type sensor, use a new oxygen sensor gasket as well.
Tighten the two nuts with the ratchet and socket or the sensor with the oxygen sensor socket and ratchet. Make sure not to over-tighten the nuts or sensor.
Plug the sensor electrical connector.
Lower the vehicle.
Replace the carpeting and passenger seat if you had to remove them.
Attach the ground battery cable using the wrench.