2001 Camry Solara catalytic converter problem

My check engine light went on a few days ago. An engine check gets me a P0420 error. At the place I took my car to the guy thinks it’s a catalytic converter problem but can’t tell which one. I have a universal one on the bottom of the car but I also have one up in the manifold as well. I don’t want to spend $1200 to replace both when it might just be the universal one that I could easily replace myself or maybe even something else. Any ideas?

Find a mechanic who can look at the signals coming out of the O2 sensors.
This will take a scanner that can plot a graph, or an oscilloscope.

Converter performance codes are typically set by the first converter.
Have your mechanic remove the converter from the car, and see if the catalyst inside has come apart.