GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Pontiac 🢒 2001 🢒 Firebird 🢒 Body and Accessories 🢒 Wiring Systems 🢒 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 CO 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 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 catalyst's 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 converters oxygen storage capacity is below an acceptable threshold.

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

    • The PCM captures the current rear HO2S rich-to-lean status.
    • The Air/Fuel ratio transitions from rich to lean, or from lean to rich, depending on the capture rear HO2S rich-to-lean status.
    • The Air/Fuel ratio transitions a second time opposite the first Air/Fuel ratio transition.
    • The PCM captures the response time of the front and rear heated oxygen sensors when the Air/Fuel ratio transitions occurred. The response time is the time during which the HO2S goes from below 300 mV to above 600 mV, and from 600 mV to below 300 mV.
    • The PCM measures the time for the rear HO2S voltage to cross a reference rich-to-lean threshold, minus the time for the front HO2S voltage to cross the same rich-to-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, P0107, P0108, P0112, P0113, P0117, P0118, P0121, P0122, P0123, P0125, P0171, P0172, P0174, P0175, P0200, P0300, P0325, P0327, P0332, P0335, P0336, P0341, P0342, P0343, P0351-P0358, P0410, P0412, P0418, P0440, P0442, P0443, P0446, P0449 , P0500, P0502, P0506, P0507, P0704, P0801, P0803, P1258, P1336, P1415, P1416, and P1441 are not set.
    • The engine is operating for more than 450 seconds.
    • The ECT is between 75°C (167°F)-120°C (248°F).
    • The IAT is between -18°C (64°F)-80°C (176°F).
    • Since the end of the last idle test, the engine speed is more than 1,000 RPM for a minimum of 37 seconds for a manual transmission
    • Since the end of the last idle test, the engine speed is more than 1,000 RPM for a minimum of 44 seconds for an automatic transmission
    • The BARO is more than 74 kPa.
    • The fuel system is operating in closed loop.

Conditions for Setting the DTC

The PCM determines that 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.

Diagnostic Aids

If the HO2S parameter is monitored using a scan tool while this diagnostic is running, the HO2S voltages may be fixed low or high.

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 the applicable DTC table

Go to Step 3

3

Inspect for the following conditions:

    • Verify that the 3-way catalytic converter is an original equipment part.
    • Inspect the 3-way catalytic converter for the following damage:
       - Dents
       - Severe discoloration from excessive temperatures
       - Holes
       - Other damage
    • Ensure that the internal 3-way catalytic converter rattle caused from a damaged catalyst is not present.
    • Inspect the exhaust system for leaks, damage, or loose or missing hardware, primarily in the area from the converter to HO2S #2. Refer to Exhaust Leakage in Engine Exhaust.
    • Ensure that the HO2S #1 and the HO2S #2 are secure and that the pigtails and the wiring are not damaged and are not contacting the exhaust.

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 - Left Side , or refer to Catalytic Converter Replacement - Right Side 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. 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