Where is the PCV valve located on a 1997 Nissan Altima?

The PCV valve is located at the rear of the engine under the intake manifold. It screws into a metal box separate from the engine and about an inch behind it. There is no way to get a wrench onto it. You can find the valve by following a vacuum hose that comes from a rail attached by the bolts that hold the manifold to the head. I don't know how Nissan techs get the thing off unless there is a special wrench. It looks like the intake manifold would have to come off. I found the thing by going to Nissan and looking at their parts diagram on their parts computer.