LEMON Manuals: Even more car manuals for everyone: 1960-2025
Home >> Chevrolet >> 2005 >> Classic >> Repair and Diagnosis >> Engine Performance >> System >> Engine Control System (DTC P0301-P2401) >> Diagnostic Information & Procedures >> DTC P0420 >> Test Description
April 5, 2026: LEMON Manuals is launched! Read the announcement.

Test Description

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

  1. 6:  This step includes inspections for conditions that can cause the three-way catalytic converter to appear degraded. Before you proceed with this table, repair any conditions that you find.
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 Observe the DTC information on the scan tool.
Are there any other component DTCs set?		 Go to Diagnostic Trouble Code (DTC) List - Vehicle in Vehicle DTC Information Go to Step 3 
3
  1. Allow the engine to reach operating temperature.
  2. Ensure Closed Loop operation is achieved.
  3. Increase the engine speed to 1,500 RPM for 1 minute.
  4. Return the engine to a stabilized idle.
  5. Monitor the heated oxygen sensor (HO2S) 1 and HO2S 2.
Is the HO2S 2 voltage parameter as active as the HO2S 1 voltage parameter?		 Go to Step 5  Go to Step 4 
4
  1. Clear the DTCs with a scan tool.
  2. Start the engine.
  3. 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 DTC P0420 set?		 Go to Step 5  Go to Diagnostic Aids
5
IMPORTANT: Verify that the three-way catalytic converter is a high quality part that meets the original equipment manufacturer (OEM) specifications.
Visually and physically inspect the three-way catalytic converter for the following conditions:
  • Dents or damage
  • Severe discoloration caused by excessive temperatures
  • Holes or punctures
  • Internal rattles caused by a damaged catalyst element
Did you find a condition?		 Go to Step 9  Go to Step 6 
6  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 10  Go to Step 7 
7 Visually inspect the catalyst monitor HO2S 2 for the following conditions:
  • The pigtail and wiring harness making contact with the exhaust or any good ground
  • Road damage

Did you find a condition?		 Go to Step 8  Go to Step 9 
8 Replace the HO2S 2 sensor. Refer to Heated Oxygen Sensor (HO2S) Replacement (Bank 1) or Heated Oxygen Sensor (HO2S) Replacement (Bank 2) .
Did you complete the replacement?		 Go to Step 10  -
9
IMPORTANT: Locate and repair the cause of the three-way catalytic convertor failure before installing the replacement converter. Refer to Diagnostic Aids.
Replace the three-way catalytic converter. Refer to Catalytic Converter Replacement (2.2L) in Engine Exhaust.Did you complete the replacement?		 Go to Step 10  -
10
  1. Clear the DTCs with a scan tool.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. 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 11 
11 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
scientia non olet · About LEMON Manuals