Diagnostic Aids

WARNING: This page is about a different variant/trim than selected.

These conditions may cause a catalytic converter to degrade. Inspect for the following conditions:
- An engine misfire
- High engine oil or high coolant consumption
- Retarded spark timing
- A weak or poor spark
- A lean fuel mixture
- A rich fuel mixture
- A damaged oxygen sensor or wiring harness
- The catalyst test may abort if the vehicle falls outside the conditions for running the DTC.
- The catalyst test may abort due to a change in engine load such as the A/C or engine cooling fan cycling.
- A catalyst may be temporarily degraded if a fuel with high sulfur content has been used. Drive the vehicle at highway speeds for 10 minutes and retest the converter.
- If 6 tests have been attempted and the DTC has not run or passed during this key cycle, turn the key to OFF for 30 seconds. Perform the Conditions for Running a second time.
- A maximum of 12 tests per key cycle will run if each test is a combination of pass, fail, or abort tests.
- After a code clear, the catalyst test will run once if the test is a pass.
- Allow the engine to return to a stabilized idle. Keep the vehicle in Drive.
If an intermittent condition cannot be duplicated, the information included in the Freeze Frame Records can be useful in determining the vehicle operating conditions when the DTC was set.
If the condition is determined to be intermittent, refer to Intermittent Conditions .

DTC P0420

Step	Action	Yes	No
1	Did you perform the Diagnostic System Check - Vehicle?	Go to Step 2	Go to Diagnostic System Check - Vehicle in Vehicle DTC Information
2	Allow the engine to reach operating temperature. Ensure Closed Loop is achieved. Increase the engine speed to 1,500 RPM for 1 minute. Return the engine to stabilized idle. Monitor the heated oxygen sensor (HO2S) 1 and the HO2S 2. Is the HO2S 2 voltage parameter as active as the HO2S 1 voltage parameter?	Go to Step 4	Go to Step 3
3	IMPORTANT: The test may need to be completed up to 6 times in order to pass or fail. If more than 6 tests have been attempted and the DTC has not passed or failed this ignition cycle, the test may be aborting. Observe the Freeze/Frame Records for this DTC. Clear the DTCs with a scan tool. Turn OFF all accessories. Operate the vehicle within the Conditions for Running this DTC. Does the scan tool indicate DTC P0420 ran and passed this ignition?	Go to Diagnostic Aids	Go to Step 4
4	IMPORTANT: Verify that the three-way catalyst (TWC) is a high quality part that meets the original equipment manufacturer (OEM) specifications. Visually and physically inspect the catalytic converter for the following conditions: Dents A severe discoloration caused by excessive temperatures Road damage An internal rattle caused by damaged catalyst substrate Restrictions-Refer to Restricted Exhaust in Engine Exhaust. Did you find a condition?	Go to Step 8	Go to Step 5
5	Visually inspect the exhaust system for the following conditions: Leaks-Refer to Exhaust Leakage in Engine Exhaust. Physical damage Loose or missing hardware The HO2S 2 for proper torque Did you find and correct the condition?	Go to Step 9	Go to Step 6
6	Visually inspect the HO2S 2 for the following conditions: The pigtail and wiring harness contacting the exhaust or a ground Physical damage Did you find a condition?	Go to Step 7	Go to Step 8
7	NOTE: Refer to Heated Oxygen Sensor (HO2S) Resistance Learn Reset Notice in Cautions and Notices. Replace the HO2S 2 sensor. Refer to Heated Oxygen Sensor (HO2S) Replacement (Pre-Catalyst) or Heated Oxygen Sensor (HO2S) Replacement (Post-Catalyst) .Did you complete the replacement?	Go to Step 9	-
8	NOTE: Refer to Three-Way Catalytic Converter Damage Notice in Cautions and Notices. Replace the catalytic converter. Refer to Catalytic Converter Replacement (3.5L Engine - Front) or Catalytic Converter Replacement (3.5L Engine - Rear) in Engine Exhaust.Did you complete the replacement?	Go to Step 9	-
9	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 10
10	Observe the Capture Info with a scan tool. Are there any DTCs that you have not diagnosed?	Go to Diagnostic Trouble Code (DTC) List - Vehicle in Vehicle DTC Information	System OK