I have replaced a few cat. convertors on the 3.5l pathfinders. I have also seen some with both bad. Generally a cat. convertor is expected to last approx. 100,000 miles. You are not likely to find an aftermarket one to fit due to the design on that engine. Used, no chance. The scrap value for used convertors is very high. They get recycled for the platinum inside. It is so valuable that people are cutting convertors off cars on dealer lots to sell them for scrap.