Hello,

My 2002 Honda Civic SI sat without running for five years. Last summer I changed the alternator and the throttle body and got it running again. Within a few months it starting throwing P0420. The car now has 195k miles. I changed the middle O2 sensor and that didn't help. Does this mean I need a new catalytic converter? Is there any thing else to check?
Thanks,
Thomas

 

Short answer yes, your cat is bad. If you're in CA, make sure you get a CARB legal one with the E.O. # stamped on it (visible to the smog tech!). The ECU is expecting a specific voltage on your Sec O2 and that is not lining up with your Pri O2 output. It is possible that your O2s are bad, but you *should* get a code for those (P0135/P0136) if they go bad, not P0420.
Keep in mind, burning oil will ruin your cat.