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Emission code P0421on 2001 Mitsubishi Galant

I have a question for you.I have an emission code #P4021-warm-up catalyst below threshold bank 1.I have replaced the upper O2 sensor but not the lower O2 sensor.The inspection guy @Jiffy Lube told me today that my catalic converter could be bad.I cleared the code with my code scanner and the check engine light stayed off for about 15 miles and the light popped back on and I have the same code again P0421 any ideas or anyone had this problem before?