I've got a used universal cat that came with some headers I bought. I have no idea on mileage as I bought it used and I've never run it. You can easily see through the core and it doesn't appear to be plugged or melted in any way. There is some light surface rust as it is used.
3" in 3" out, but the previous owner welded a reducer to 2.25" on the back that can easily be cut off. Doesn't appear to be a brand name stamped on it, only the numbers 94000. Typing that into Google brings up Magnaflow's 94000 series cats, but I'm not sure if it's a Magnaflow or not.
Asking $20 + shipping for it.
