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HOMESERVICE MANUALSCHEVROLET2006COBALT SS, 4D SEDAN, STANDARDREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 383 (ENGINE CONTROL SYSTEM - 5.3L - DTC P0136 TO DTC P0446)DTC P0420TEST DESCRIPTION
2006 Chevrolet Cobalt SS, 4D Sedan, Standard

Test Description

2006 Chevrolet Cobalt SS, 4D Sedan, StandardSECTION Test Description
WARNING: This page is about a different car, the 2006 Chevrolet Monte Carlo and 2006 Chevrolet Impala. However, it is still accessible from the selected car via links, so may be relevant.

The numbers below refer to the step numbers on the diagnostic table.

  1. 5:  A catalytic converter which has been discolored may be due to an engine running rich, lean, or had a previous misfire. Verifying the fuel trim percentages may be of assistance in determining if such a condition exists.
  2. 6:  This step inspects for conditions than can cause the TWC efficiency to appear degraded.
DTC P0420

Step Action Values Yes No
1 Did you perform the Diagnostic System Check - Vehicle? - Go to Step 2  Go to Diagnostic System Check - Vehicle
2 Review the DTC information on the scan tool.
Are any other DTCs set?		 - Go to Diagnostic Trouble Code (DTC) List - Vehicle Go to Step 3 
3
IMPORTANT: A new converter with less than 100 miles on it may set DTC P0420 or P0430 due to out-gassing of the internal matting. Operating the vehicle at highway speeds for approximately 1 hour may correct the condition.
  1. Start and idle the engine.
  2. Allow the engine to reach operating temperature.
  3. Increase the engine speed to 2,000 RPM for 2 minutes.
  4. Ensure Closed Loop operation is enabled.
  5. Return the engine to a stabilized idle.
  6. Observe the HO2S 2 voltage parameter on the scan tool.
Is the HO2S 2 voltage parameter transitioning below the first specified value and above the second specified value?		 300 mV
650 mV		 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 3-way catalytic converter (TWC) is a high quality part that meets the OEM specifications.
Visually and physically inspect the TWC for the following conditions:
  • Physical damage
  • Severe discoloration caused by excessive temperatures
  • Internal rattles caused by loose catalyst substrate
  • Restrictions-Refer to Restricted Exhaust .
Did you find and correct the condition?		 - Go to Step 10  Go to Step 6 
6  Visually inspect the exhaust system for the following conditions:
  • Leaks-Refer to Exhaust Leakage .
  • 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 10  Go to Step 7 
7 Visually inspect the HO2S 2 for the following conditions:
  • The pigtail and wiring harness contacting the exhaust or any 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 Sensor 2 .
Did you complete the replacement?		 - Go to Step 10  -
9
NOTE: In order to avoid damaging the replacement three-way catalytic converter, correct the engine misfire or mechanical fault before replacing the three-way catalytic converter.
Replace the TWC. Refer to Catalytic Converter Replacement (RPOs LZE/LZ4/LZ9) or Catalytic Converter Replacement (RPO LS4) .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.
    CAUTION: Refer to Road Test Caution .
    RENDER: 1.0x

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    When to See a Mechanic

    Stop DIY work and contact a certified mechanic immediately if any of the following apply:

    • • You smell fuel, burning insulation, or see smoke.
    • • Brakes feel soft, pull hard to one side, or make grinding noises.
    • • The engine overheats, stalls repeatedly, or misfires under load.
    • • You are missing required tools, torque specs, or safe lifting equipment.
    • • You are not confident in the next step or safety outcome.