Driving a 99 civic si with 170k on odometer back in march I installed a new cat its an oem style eastern catalytic cat i purchased for around 120 bucs. Just this week I got a p0420 code. I don't understand could this cat be bad already. When i first installed it I got a o2 sensor code which i cleared and never came back.

My Mechanic who works for honda for over 20 years and is a good friend said its definitely a bad cat. Anyone else ever run into this.

 

replace the o2 sensor

 

Have been told if the o2 sensor was the problem it would throw that code not the cat code. I'm confused.