Hardy,
They are there to make the rig smog legal. Aftermarket ones are usually sold based on the cubic inches of the engine. If you have a O2 sensor on yours, make sure the replacement converter has a fitting in it for this.
Are you indicating that the converters you are looking at are USED!! If that's what you are saying, I wouldn't buy them. These items are considered a consumable..they wear out!
Stuart
