the situation is i took off the front o2 sensor and i saw it melt at that time i got the p0420 code(bank1) and i bought the new one NGK o2 sensor and install it after install i reset the ECU but the code come back after the car run 100 miles and i thought it might be the second one so i took it off to check it and nothing wrong with it ( in the rear o2 sensor i got the o2 fix already ) i never get the code since i have my car . i just dont know why it still come back because i already replace the new one and reset the ECU but it stilli went to the shop and both o2 sensor work fine . And somebody told me because the front o2 sensor somebody told me the second o2 sensor so i don't know which one is it and i don't know what happen to my car

any idea ?, i really need help. this code make me crazy i tried to fix it but it still

thanks a lot