Yes it is a catalytic converter and yes the pipe is the same diameter as the pipe on the car but that's it. I wouldn't call it a spare, it's no more than 1/4 the volume of the converter that was removed from the car. It's a lot shorter so I had to buy more tubing to install it and there's no O2 sensor port. Okay, too bad I didn't notice that in the description, but seriously, what cat doesn't have an O2 sensor these days? Also, the factory converter has the input and output offset from each other to make room for the O2 sensor, which means I have to weld a number of angles into the tube. It was fun. It works, but be prepared for a lengthy installation and don't expect top performance.