How to Diagnose a P0420 Code

Locate the suspect catalytic converter to be tested - Watch

Tip : Only catalytic converters monitored by o2 or air/fuel ratio sensors can set a P0420 code

Check exhaust system for leaks by visually inspecting for cracked or rusted through exhaust components. With the engine cold, start the vehicle and listen for exhaust leaks paying close attention to pipe connections and flex pipes - Watch
If a hard-to-find leak is present, pin-point leak using soapy water and a shop vac - Watch

Tip : For best results test for leaks when exhaust system is cold

Start the vehicle and bring the engine up to operating temperature, this ensures the catalytic converter is hot enough to function properly - Watch
Using a scan tool or a lab scope graph the o2 sensor signals upstream and downstream of the catalytic converter - Watch

Tip : some vehicles use an air/fuel ratio sensor before (upstream) the catalytic converter which will not switch like an o2 sensor when graphed, but the downstream o2 sensor activity can still be monitored for testing

Monitor o2 sensor signals at idle and at 2000 rpm, observing the switch rate from rich to lean on the downstream o2 sensor signal. If the catalytic converter is operating properly the signal should be relatively inactive and straight on the graph - Watch

Tip : If the vehicle is equipped with an upstream o2 sensor, compare the upstream to downstream o2 sensors and look for the upstream sensor to be much more active than the downstream. If both sensors have close to the same switch rate it is likely the catalytic converter has failed

Test o2 sensor for proper operation to confirm tests results are accurate - Watch
How to test an oxygen sensor
If testing confirmed a failed catalytic converter, replace with new quality replacement part - Watch
After replacement confirm proper operation of new catalytic converter - Watch