96 Lumina 3.1 P0420

[weidnerr]

What all can I check before changing the cat out? I recently changed the front O2 sensor but not the back. I thinking of trying the back O2 sensor first.
It has recently been tuned up. Any suggestions?

[maxwedge]

This can be a tricky code to correct as many issues can cause this. A graphing scanner properly utilized can check cat efficency, another way is to check the inlet vs outlet temp of the cat when fully warmed up, there should be at least 100 degrees difference, the outlet is hotter. Misfires, lean mixtures, bad post cat sensors or the cat itself can cause this code.