Emission Control Systems Diagnosis
The major systems
Know PCV, EVAP, EGR, secondary air, the catalyst, and the O2 sensors, and know what each failure looks like. A stuck-open PCV valve leans idle; a plugged one pressurizes the crankcase and pushes seals out. A purge valve stuck open causes rich idle and rough running, while a small EVAP leak (about 0.020 inch) needs a smoke test or a scan-tool leak test, and the vent valve must close for that test to run. Too much EGR flow causes rough idle and stall; too little causes pinging and high NOx. An AIR pump check valve failure lets exhaust back into the pump and can produce a false lean reading during open loop.
Catalyst efficiency
P0420 is an efficiency code, but exhaust leaks, a lazy downstream O2 sensor, or a history of misfire can set it too. Verify with downstream O2 behavior and inlet versus outlet temperature before condemning the converter.
Traps
Replacing a converter for P0420 without checking misfire history, fuel trims, and exhaust leaks; and chasing an EVAP leak without checking the fuel cap and filler neck first.
Rule of thumb
A healthy converter stores oxygen. The downstream O2 should sit fairly steady around 0.6-0.7 V while the upstream sensor switches rapidly. If the rear sensor mirrors the front, the converter has lost oxygen storage.