GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2001 🢒 Tahoe - 4WD 🢒 Engine 🢒 Engine Controls - 8.1L 🢒 DTC P0420 or P0430

Circuit Description

In order to maintain a 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 CO present in the exhaust gas. This reaction converts them into harmless water vapor and carbon dioxide. The catalyst also reduces NOx, converting it to nitrogen. The powertrain control module (PCM) monitors this process using post-catalyst heated oxygen (HO2S) sensors. The post-catalyst heated oxygen sensors, located in the exhaust stream past the 3-way catalytic converter, produce an output signal which indicates the oxygen storage capacity of the catalyst. This determines the catalysts ability to convert exhaust emissions effectively. If the catalyst is functioning correctly, the post-catalyst sensor activity will be far less active than that produced by the pre-catalyst sensors. This indicates that the 3-way catalytic converters oxygen storage capacity is operating at a threshold considered acceptable.

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

  1. The powertrain control module (PCM) captures the current rear HO2S rich or lean status.
  2. The air fuel ratio transitions from rich to lean or lean to rich depending on the capture rear HO2S rich or lean status.
  3. The air/fuel ratio transitions a second time opposite the first air/fuel ratio transition.
  4. 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 heated oxygen sensors when the air fuel ratio transitions occurred.
  5. The PCM measures the time it takes the rear HO2S voltage to cross a reference rich or lean threshold minus the time it takes 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 exceeds a predetermined threshold.

Conditions for Running the DTC
    • DTCs P0101, P0102, P0103, P0106, P0107, P0108, P0112, P0113, P0117, P0118, P0125, P0128, P0131, 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 for 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 to +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 which 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 3-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
    • Insure that the internal 3-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
    • Insure that the pre-catalyst sensors and the post-catalyst sensors are secure and that the pigtails and 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.

Is the action complete?

Go to Step 5

--

5
  1. Use the scan tool in order to clear the DTCs.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.

Does the DTC run and pass?

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