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It's actually very hard to even see. To replace the sensor the intake manifold has to be removed. Once you remove the intake manifold you will be able to see it much better. The sensor is located under the intake manifold (which you must remove), left side (driver's side) between the last two cylinders on that side.
Did you take the upper and lower intake manifold piece off?
Did you look on the left side between the two cylinders? (Cylinders 4 and 6)
Also did you look at the pic of the sensor to know what your looking for.
Can you take a look at these picture? and point me to the correct loction on the Nissan pathfinder 1999
You only took of the top part low intake manifold also has to come out. This is the actual location of the sensor.