P0420 even after Cat replacement

[300kmi97camry]

I have a 97 camry LE with 309000 milesthe cat an O2 sensors have never been replace. Tell me what you think. I was getting code P0420 for months, actually i was getting a p0420 code and a pending p0420. My brother-in-law knows how to weld so I decided to get a universal magnaflow, so we take down the cat system, cut off the old one and weld the new one back on. I am now getting the p0420 code and pending p0420. I just erase it every other day or so with my scanner. Should I look closer at my O2 sensors now?

[camlex]

I had same problem, i think you should know which one to use depends on flow.
finally, somebody install from orailly"walker" brand.

Error code was @286K now i have 351,800 with same cat. No problem !!!

[300kmi97camry]

thanks camlex, I'll look into walker brand. the magnaflow was only 60 something bucks. I guess i can cut it off again and weld another one back on.

[hill8570]

300K+ on the original O2 sensors? I think you're about 200K overdue to replace them (IMHO).

If it were me, I'd throw an oscilloscope on the O2 sensor downstream of the cat (that's the beast that triggers the P0420), and see if it matches the waveform in the FSM. Or, you can just shotgun the sensor and see if the code goes away.