What kind of rear catalytic converter do I need for a 2003 Nissan Altima 4cyl 2.5L

I would suggest we start with testing the converter unless you have a code that would point to a converter issue.

Here are some guides that will help with both of these items:

https://www.2carpros.com/articles/how-to-test-a-catalytic-converter

If you have not checked for codes, then I would suggest we start with that.

https://www.2carpros.com/articles/checking-a-service-engine-soon-or-check-engine-light-on-or-flashing

However, if you are already past all this and just know you need the converter then here is the info on how to replace that including the part number.

The only thing I will clarify though is the rear converter is most likely the front exhaust tube. I am attaching all the info on this.

Let me know what questions you have, and we can go from there.

Thanks