DTC P0420 - Three-Way Catalyst (TWC) Efficiency Low

Connect scan tool to Data Link Connector (DLC). Turn ignition on. Clear DTCs. Select READINESS TESTS mode on scan tool to monitor catalyst. Test drive vehicle at 35-45 MPH in 3rd gear (A/T or M/T) for at least 8 minutes. Slow vehicle speed to 30-40 MPH and hold for at least 2 minutes. Stop vehicle, leaving engine running. Review test results. If DTC P0420 resets, go to next step. If TEST COMPLETED is displayed on scan tool, and no DTC is set, test is complete.
If short term fuel trim varied between -20 and +20 percent while test driving vehicle, go to next step. If short term fuel trim was not as specified, check fuel system following procedures in DTCs P0171 and P0172.
Select DATA LIST mode on scan tool. Test drive vehicle under normal driving conditions for 5 minutes. Check HO2S No. 2 output voltage and short term fuel trim. Record readings. Stop vehicle, leaving engine running. Accelerate vehicle to greater than 20 MPH and come to a stop. Repeat 9 times. Go to next step.
Increase vehicle speed to 50 MPH in 3rd gear. Release accelerator and allow vehicle to coast for 10 seconds or more. Stop vehicle and let idle for 2 minutes. Test is complete. If HO2S No. 2 output voltage varied from less than 0.3 volt to greater than 0.6 volt during test, replace TWC. If HO2S No. 2 output voltage was not as specified, check Light Green/Black wire and Yellow wire between HO2S No. 2 and PCM for an open or short. Repair as necessary. If wires are okay, replace HO2S No. 2. Retest system.