Test Description

The number below refers to the step number on the diagnostic table.

5: The MIL flashes if the ECM detects a catalyst damaging misfire. If the engine is misfiring, repair the misfire condition so catalyst damage does not occur.

DTC P0420

Step	Action	Yes	No
1	Did you perform the Diagnostic System Check - Engine Controls?	Go to Step 2	Go to Diagnostic System Check - Engine Controls
2	Observe the DTC information on the scan tool. Are there any other DTCs set?	Go to Diagnostic Trouble Code (DTC) List	Go to Step 3
3	Inspect for the following conditions: The catalytic converter is an original part. Inspect the exhaust system for leaks, damage, or missing hardware in the area from the converter to the heated oxygen sensor (HO2S) 2. HO2S 2 is secure and the wiring is not damaged or contacting the exhaust. Did you find and correct the condition?	Go to Step 5	Go to Step 4
4	IMPORTANT: Before replacing the 3-way catalytic converter (TWC), correct any conditions that may have damaged the converter. Replace the three-way catalytic converter. Refer to Catalytic Converter Replacement in Engine Exhaust.Did you complete the replacement?	Go to Step 5	-
5	Was the customer's concern that the malfunction indicator lamp (MIL) flashes?	Go to DTC P0300	Go to Step 6
6	Clear the DTCs with a scan tool. Turn OFF the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records. Did the DTC fail this ignition?	Go to Step 2	Go to Step 7
7	Observe the Capture Info with a scan tool. Are there any DTCs that you have not diagnosed?	Go to Diagnostic Trouble Code (DTC) List	System OK