need help CEL came on after tune up

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Junior Member
Hi, I got a honda civic 99 dx from a friend of mine. She had the car tune up by a dealership and after 3 days the check engine light came on. I used a code reader and it came up as "P0420" Catalyst System Efficiency below threshold (bank 1), I erase it using the code reader and she went ahead and have the car smog checked and it pass with flying colors , now I got the car after 5 days the CEL came on again and it reads the same "P0420".
I read somewhere that P0420 means the catalytic converter is bad, but if it is bad, it shouldn't have pass smog or will it pass still with a bad catalytic converter? will a bad O2 sensor give me this code?

I just check the connection going to the O2 sensor both primary and secondary and the primnary have about 12 volts on the heater lead but on the secondary it doesn' have any voltage. This is with the ignition on and the car is cold didn't even try to run it and the CEL is still on I dodn't clear it.
Does both heater lead that comes from the PCM have to have 12 volts or is it just the primary?

any help or suggestion as to how to trouble shoot this problem is greatly appreciated.

BTW the car is running good and fuel efficiency is still the same.