GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2002 🢒 Express 🢒 Engine 🢒 Engine Controls - 8.1L 🢒 DTC P0420 or P0430

Circuit Description

In order to maintain reasonably low emissions of hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx), the engine controls system uses a 3-way catalytic converter. The catalyst within the converter promotes a chemical reaction which oxidizes the HC and the CO present in the exhaust gas. This reaction converts these chemicals into harmless water vapor and carbon dioxide. The catalyst also reduces NOx, converting the NOx to nitrogen. The powertrain control module (PCM) monitors this process using bank 1 heated oxygen sensor (HO2S) 2. Bank 1 HO2S 2, located in the exhaust stream past the 3-way catalytic converter, produces an output signal which indicates the oxygen storage capacity of the catalyst. This determines the catalysts ability to effectively convert the exhaust emissions. If the catalyst is functioning correctly, the bank 1 HO2S 2 signal will be far less active than the signal produced by bank 1 HO2S 1. This indicates that the 3-way catalytic converter's oxygen storage capacity is operating at an acceptable threshold.

When the conditions for running this DTC are met, the following conditions occur:

    • The PCM captures the current rear HO2S rich or lean status.
    • The air/fuel ratio (A/F) transitions from rich to lean or lean to rich depending on the capture rear HO2S rich or lean status.
    • The A/F transitions a second time opposite the first A/F transition.
    • The PCM captures the response time, the time the HO2S goes from below 300 mV to above 600 mV and from 600 mV to below 300 mV, of the front and rear HO2S when the A/F transitions occurred.
    • The PCM measures the time needed in order for the rear HO2S voltage to cross a reference rich or lean threshold. Next, the PCM subtracts the time needed in order for the front HO2S voltage to cross the same rich or lean threshold. The time difference from the front and rear HO2S is the oxygen storage capacity of the catalyst. This DTC sets if the time used exceeds a predetermined threshold.

Conditions for Running the DTC
    • DTCs P0101, P0102, P0103, P0106, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0125, P0128, P0131 or P0151, P0132, P0133, P0134, P0135, P0137, P0138, P0140, P0141, P0151, P0152, P0153, P0154, P0155, P0157, P0158, P0160, P0161, P0171, P0172, P0174, P0175, P0200, P0300, P0335, P0336, P0351-P0358, P0341, P0342, P0343, P0404, P0405, P0410, P0418, P0443, P0446, P0449, P0500, P0502, P0503, P1120, P1125, P1133, P1134, P1153, P1154, P1220, P1221, P1404, P1415, or P1416 are not set.
    • The engine has been operating more than 600 seconds.
    • The engine coolant temperature (ECT) is between 70-158°C (176-248°F).
    • The intake air temperature (IAT) is between -15°C and +75°C (5-167°F).
    • The barometric pressure (BARO) is more than 74 kPa.
    • The fuel system is operating in Closed Loop.

Conditions for Setting the DTC

The PCM determines the oxygen storage capability of the catalytic converter has degraded below a calibrated threshold.

Action Taken When the DTC Sets
    • The control module illuminates the malfunction indicator lamp (MIL) when the diagnostic runs and fails.
    • The control module records the operating conditions at the time the diagnostic fails. The control module stores this information in the Freeze Frame/Failure Records.

Conditions for Clearing the MIL/DTC
    • The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
    • A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
    • A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
    • Clear the MIL and the DTC with a scan tool.

Test Description

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

  1. Before replacing the 3-way catalytic converter, correct any conditions that may have damaged the catalyst.

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

Are any other DTCs set?

Go to Diagnostic Trouble Code (DTC) List

Go to Step 3

3
  1. Inspect for the following conditions:
    2. • Verify that the three-way catalytic converter is an original equipment part.
    • Inspect the 3-way catalytic converter for damage. Inspect for the following conditions:
       - Dents
       - Severe discoloration caused by excessive temperatures
       - Holes
    • Verify that the internal three-way catalytic converter rattle caused by damaged catalyst is not present.
    • Inspect the exhaust system, primarily in the area from converter to HO2S 2, for the following conditions:
       - Exhaust leaks--Refer to Exhaust Leakage in Engine Exhaust.
       - Damage
       - Loose or missing hardware
    • Verify that the HO2S 1 and the HO2S 2 are secure and that the pigtails and the wiring are not damaged or contacting the exhaust. Refer to Heated Oxygen Sensor Wiring Repairs in Wiring Systems.
  2. Repair conditions as necessary.

Did you find and correct the condition?

Go to Step 5

Go to Step 4

4

Replace the catalytic converter. Refer to Catalytic Converter Replacement in Engine Exhaust.

Did you complete the replacement?

Go to Step 5

--

5
  1. Use the scan tool in order to clear the DTCs.
  2. Idle the engine at the normal operating temperature.
  3. Select the Diagnostic Trouble Code (DTC) option.
  4. Select the Specific DTC option, and input the DTC number into the scan tool.
  5. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text, if applicable.

Does the scan tool indicate that this test ran and passed?

Go to Step 6

Go to Step 2

6

With a scan tool, observe the stored information, Capture Info.

Does the scan tool display any DTCs that you have not diagnosed?

Go to Diagnostic Trouble Code (DTC) List

System OK