GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 2008 🢒 SLS 🢒 DTC 🢒 DTC P0420 or P0430

Circuit Description

In order to maintain a reasonably low emission level of hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx) a 3-way catalytic converter (TWC) is used. The catalyst within the converter promotes a chemical reaction that oxidizes the HC and CO present in the exhaust gas. This reaction converts the gases into harmless water vapor and carbon dioxide. The catalyst also reduces the NOx , converting the NOx into nitrogen. The engine control module (ECM) monitors this process using the post catalyst heated oxygen sensor (HO2S) signal. The post-catalyst HO2S located in the exhaust stream after the TWC, produces an output signal that indicates the oxygen storage capacity of the catalyst. The oxygen storage capacity (OSC) determines the ability of the catalyst to convert the exhaust emissions effectively. If the catalyst is functioning correctly, the post-catalyst HO2S signal will be far less active than the signal produced by the pre-catalyst HO2S.

To determine OSC, the ECM commands a rich air/fuel mixture until all oxygen is removed from the catalyst. The ECM then commands a lean air/fuel mixture and monitors the rear heated oxygen sensors to calculate the oxygen storage capacity. The catalyst is operated in this mode until one of the following conditions occur:

    • The oxygen stored in the catalyst exceeds a calibrated threshold, which is determined from the rear HO2S signal.
    • The rear HO2S indicates the catalyst to be completely saturated with oxygen, which is determined from the rear HO2S signal.

If the ECM detects the average OSC is less than a threshold, this DTC sets. This indicates that the TWC oxygen storage capacity is below a threshold considered acceptable.

DTC Descriptors

This diagnostic procedure supports the following DTCs:

    • DTC P0420 Catalyst System Low Efficiency Bank 1
    • DTC P0430 Catalyst System Low Efficiency Bank 2

Conditions for Running the DTC
    • Before the ECM can report DTC P0420 or P0430 failed, DTCs P0030, P0031, P0032, P0036, P0037, P0038, P0050, P0051, P0052, P0053, P0056, P0057, P0058, P0059, P0101, P0102, P0103, P0121, P0122, P0123, P0130, P0131, P0132, P0133, P0135, P0137, P0138, P0140, P0141, P0150, P0151, P0152, P0153, P0155, P0157, P0158, P0160, P0161, P0221, P0222, P0223, P0335, P0336, P0338, P167A, P167B, P2096, P2097, P2098, P2099, P2195, P2196, P2197, P2198, P2232, P2235, P2237, P2240, P2243, P2247, P2251, P2254, P2270, P2271, P2272, P2273, P2297, P2298, P2626, and P2629 must run and pass.
    • DTCs P0010, P0011, P0013, P0014, P0020, P0021, P0023, P0024, P0030, P0031, P0032, P0036, P0037, P0038, P0050, P0051, P0052, P0053, P0056, P0057, P0058, P0059, P0101, P0102, P0103, P0116, P0117, P0118, P0119, P0121, P0122, P0123, P0125, P0130, P0131, P0132, P0133, P0135, P0137, P0138, P0140, P0141, P0150, P0151, P0152, P0153, P0155, P0157, P0158, P0160, P0161, P0221, P0222, P0223, P0300, P0301-P0306, P0443, P0458, P0459, P0496, P167A, P167B, P2088, P2089, P2090, P2091, P2092, P2093, P2094, P2095, P2096, P2097, P2098, P2099, P2100, P2101, P2107, P2119, P2122, P2123, P2127, P2128, P2138, P2176, P2177, P2178, P2179, P2180, P2187, P2188, P2189, P2190, P2195, P2196, P2197, P2198, P2232, P2235, P2237, P2240, P2243, P2247, P2251, P2254, P2270, P2271, P2272, P2273, P2297, P2298, P2626, and P2629 are not set.
    • The amount of exhaust emissions entering the engine is within the allowable range.
    • The ambient temperature is more than -30°C (22°F).
    • The engine is running for more than 450 seconds.
    • The rear heated oxygen sensor operaties continuously, reaches and exceeds the set time.
    • The engine is running in closed loop.
    • The calculated TWC temperature is within the allowable range.
    • The engine speed is within the allowable range.
    • The engine load is within the allowable range.
    • Rear HO2S is ready.
    • The EVAP solenoid leak inspect is inactive.
    • Front HO2S respond has set.
    • The short term fuel trim value is in the predetermined range.
    • The three-way catalytic converter clear function is not set.
    • The three-way catalytic converter monitors equipment controls command.
        OR
    • The difference between the predetermined TWC Temperature and the filtered actual TWC Temperature is less than 30°C (86°F).
    • The difference between the expected exhaust emissions flow and the filtered exhaust emissions flow is less than 11.11 g/s.
    • The fuel system is in Closed Loop.
    • The misfire times did not exceed the specified one caused by damaged catalyst.
    • The quick fuel mixed adapt feature is not completed.
    • The triggering condition changes according to the setting steps.
    • The oxygen storage capacity (OSC) measured is less than the specified value.
    • Set the filtered oxygen storage capacity (OSC) less than the specified value.
    • The above conditions exist for approximately 1,000 seconds.
    • DTCs P0420 and P0430 run once a drive cycle. The ECM will attempt to run this diagnostic up to 3 times a drive cycle.

Conditions for Setting the DTC

The ECM determines that the catalyst efficiency has degraded below a calibrated threshold for more than 4 seconds.

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 Failure Records.

Conditions for Clearing the MIL/DTC

    • The control module turns OFF the malfunction indicator lamp (MIL) after 4 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.

Step

Action

Yes

No

Schematic Reference: Engine Controls Schematics

Connector End View Reference: Engine Control Module Connector End Views or Engine Controls Connector End Views

1

Did you perform the Diagnostic System Check - Vehicle?

Go to Step 2

Go to Diagnostic System Check - Vehicle

2

Are any other DTCs set?

Go to Diagnostic Trouble Code (DTC) List - Vehicle

Go to Step 3

3

Inspect for the following conditions:

    • The catalytic converter is an original equipment part.
    • Inspect the exhaust system for leaks, damage, or loose or missing hardware in the area from the converter to the heated oxygen sensor (HO2S) 2.
    • The HO2S 2 is secure and the wiring is not damaged or contacting the exhaust.

Did you find and correct the condition?

Go to Step 5

Go to Step 4

4

Important: Before replacing the 3-way catalytic converter (TWC), correct any conditions that may have damaged the converter.

Replace the catalytic converter. Refer to the appropriate procedure:

Did you complete the replacement?

Go to Step 5

--

5
  1. Clear any 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 6

6

Observe the Capture Info with a scan tool.

Are there any DTCs that have not been diagnosed?

Go to Diagnostic Trouble Code (DTC) List - Vehicle

System OK