Check engine light is on and code P0420

i take it this is a O2 sensor code.. well, according what i learn in my Honda OBD2 class that's an air fuel ratio sensor not Oxygen sensor. only way to diagnose if it is a fault is using a Honda manufacture scanner. watch live data and mode 6. if it is a true Oxygen sensor code then it has to be the front. rear sensor is responsible for catalytic converter code.. anymore question? by the way a diagnostic code only means computer sense either sensor or components losing power/ground or the value is out of range as what computer expect. it didn't say the sensor is no good.

Good afternoon,

You will need a scan tool that can read the voltage of the upstream and downstream O2 sensors.

https://www.2carpros.com/articles/how-to-test-a-catalytic-converter

When the readings vary the same, that indicates the cat converter is bad. If the downstream sensor voltage stays stable, then it could be an exhaust leak in the system.

In most cases it is the cat converter, not the sensors. If they were bad, they would code separately for the sensors.

Roy