Diagnostic Aids

WARNING: This page is about a different car, the 2004 Pontiac Grand Prix. However, it is still accessible from the selected car via links, so may be relevant.

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.
- If 18 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.
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.
The catalyst may have been temporarily contaminated with a chemical from a fuel additive, fuel contamination, or any of the above conditions.

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-Engine Controls?	Go to Step 2	Go to Diagnostic System Check - Engine Controls
2	Allow the engine to reach operating temperature. Ensure Closed Loop is achieved. Increase the engine speed to 1,500 RPM. Return the engine to a stabilized idle. Monitor the HO2S 1 and the HO2S 2 voltage parameters on the scan tool. Is the HO2S 2 voltage parameter as active as the HO2S 1 voltage parameter?	Go to Step 4	Go to Step 3
3	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. 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. Refer to Diagnostic Aids. 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 catalytic (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 heated oxygen sensor (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	Replace the HO2S 2 sensor. Refer to Heated Oxygen Sensor Replacement - Position 2 . 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 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	System OK