Do not throw parts

The downstream oxygen sensor looks too much like the upstream one. The computer thinks the catalytic converter on bank 1 is not cleaning the exhaust. That can be a dying brick — or an exhaust leak, a lazy upstream sensor, or a misfire that poisoned the cat.

Do not throw parts · Do not throw a catalytic converter from P0420 alone. A converter is $800–$2,500; a sensor or a leak is not. Verify no misfires, no exhaust leaks, and graph both O2 sensors before anyone orders a brick.

First look · Graph upstream vs downstream O2. Leak check. Clear misfires first. Then the brick.

Say this

I have P0420 stored. I want freeze-frame and a leak check before anyone prices a converter.