I have a 2003 nissan xterra and when I crank it up a loud knocking noise can be heard I do mean loud. I took it to autozone a while back when the check engine light was on and they said I needed a new knock sensor and a oxy sensor. The noise is incredibly loud and I was told it could possibly be something to do with a gasket in the manifold or something? I don't know much about cars so if anyone could help it sure would be nice

SOURCE: 2001 Nissan Xterra Knock sensor Took

it is a single wire sensor under the lower intake manifold. they are common to fail. it is a pretty big job to do. if you have the upper intake manifold then you need to remove the fuel injection rail and remove the lower intake manifold requiring new lower gaskets and the coolant will also drain to this level. watch for coolant draining into the intake ports of the heads that can cause hydro-locking. once the lower intake manifold is removed you will see the sensor right on top of the engine block about 3/4 the way back. good luck....

SOURCE: nissan xterra knock sensor

It's the valley of the engine underneath the intake manifold. It's about an $800 job because the top of the engine has to come off. However, unless you have a supercharged xterra it shouldn't effect performance. Also, the knock sensor will not trip the check engine light, as it does not indicate a primary problem, so if your light is on you should have another code that should be fixed first.