To find the other o2 sensor I need to know weather its a 6 or 4 cylinder. If its a 4 cylinder it is located on the exhaust manifold at the collector, after you find that one go for a drive and get it warm before you put a wrench on it. If its a 6 cylinder you have to follow the y pipe from the two cylinder banks down to the back of the engine compartment you may be able to see this one from the top. If you can't see it from under the hood it should be right where the two pipes come together. After removing the old one you may be able to reuse the sensor buy carefully pouring a small amount of gas on it to burn of excess carbon build up on the old sensor. Usually the sensor before the converter goes bad due to carbon build up. One last question do you know how many miles are on the van.