Need to know diagnostic code P0420 and what to do calls for Catalyst system .

Monitor downstream O2 sensor with scanner.Reads higher cold lower warm,should not switch like upstream.poss.02 faulty.

COMPONENT INSPECTION Monitor the Catalyst Converter Clear DTC. Start the engine and warm it up until the radiator fan comes on (More than at least 10 minutes ). Drive at a steady speed between 45-55 mph (72-88 km/h) for 30 seconds . Stop and then maintain idle state for 120 seconds in D-position. Repeat step 3 once again. Stop and then keep it in idle state (D-position) for 120 seconds . Repeat step 3 once again. Stop and then maintain idle state for 120 seconds in D-position. IG "OFF" Repeat steps 3 through 9 three times. Check if catalyst monitoring readiness is complete. so, if the readiness is incomplete, repeat steps 2 through 10. Does the scan tool show DTC P0420? YES - Substitute with a known - good Catalyst Converter and check the signal waveform of HO2S for proper operation. If the problem is corrected, Go to "Verification of Vehicle Repair" procedure. NO - It was intermittent failure.