i had this same problem...rough idle and a code of 21 would set. i ordered the part from the same website, and was amazed at how quickly it got to me over here in rhode island. anyway, this is one of the easiest fixes on the svx. your were looking at the right diagram. it is part number 8...sticking out of the water pipe. if you are standing over your engine on the driver's side (with the throttle body cover off) look down on the aft end of the manifold. the sensor is right there sticking up with an electrical connector on the top of it. mine was totally corroded and therefore not doing what it was intended to.
hopefully you'll have the same luck and this will fix your problem.