GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Buick 🢒 2008 🢒 LaCrosse 🢒 Safety and Security 🢒 Supplemental Inflatable Restraints 🢒 DTC P0420

Diagnostic Instructions
    • Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
    • Review Strategy Based Diagnosis for an overview of the diagnostic approach.
    •  Diagnostic Procedure Instructions provides an overview of each diagnostic category.

DTC Descriptors

DTC P0420: Catalyst System Low Efficiency Bank 1

Circuit/System Description

In order to maintain a 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 sensor 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.

Conditions for Running the DTC
    • Before the ECM can report DTC P0420 failed, DTCs P0030, P0031, P0032, P0036, P0037, P0038, P0101, P0102, P0103, P0121, P0122, P0123, P0130, P0131, P0132, P0133, P0135, P0137, P0138, P0140, P0141, P0221, P0222, P0223, P0335, P0336, P0338, P167A, P2096, P2097, P2195, P2196, P2231, P2232, P2237, P2243, P2251, P2270, P2271, P2297, and P2626, must run and pass.
    • DTCs P0010, P0011, P0013, P0014, P0020, P0021, P0023, P0024, P0030, P0031, P0032, P0036, P0037, P0038, P0101, P0102, P0103, P0111, P0112, P0113, P0117, P0118, P0119, P0121, P0122, P0123, P0128, P0130, P0131, P0132, P0133, P0135, P0137, P0138, P0140, P0141, P0221, P0222, P0223, P0300, P0443, P0458, P0459, P0496, P2088, P2089, P2090, P2091, P2092, P2093, P2094, P2095, P2096, P2097, P2100, P2101, P2107, P2119, P2122, P2123, P2127, P2128, P2138, P2176, P2177, P2178, P2187, P2188, P2195, P2196, P2232, P2237, P2243, P2251, P2270, P2271, P2297, or P2626 are not set.
    • The engine speed is 1,100-3,520 RPM.
    • The engine load is 13-55 percent.
    • The air flow into the engine is between 4-17 g/s.
    • The engine intake air temperature (IAT) at engine start-up is greater than -30°C (-22°F).
    • The HO2S 2 is at operating temperature for greater than 300 seconds.
    • The engine is operating in Closed Loop.
    • The calculated TWC temperature is between 480°C (896°F) and 750°C (1,382°F) and steady.
    • The above conditions exist for approximately 17 minutes.
    • DTC P0420 runs once per ignition cycle when the above conditions exist for approximately 17 minutes. The ECM will restart the test up to 4 times per ignition cycle, if the conditions above are interrupted during the test.

Conditions for Setting the DTC

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

Action Taken When the DTC Sets

DTC P0420 is a Type A DTC.

Conditions for Clearing the DTC

DTC P0420 is a Type A DTC.

Diagnostic Aids
    • The following conditions may cause a catalytic converter to degrade:
       - An engine misfire
       - High engine oil or high coolant consumption
       - Retarded spark timing
       - A weak or poor spark
       - A lean fuel mixture
       - A rich fuel mixture
       - A damaged oxygen sensor or wiring harness
    • To help determine if the catalytic converter is degraded, operate the engine at 1,500 RPM for 1 minute. Then allow the engine idle to stabilize. The HO2S 2 parameter should not be as active as the HO2S 1 parameter.

Reference Information

Schematic Reference

Engine Controls Schematics

Connector End View Reference

Component Connector End Views

Electrical Information Reference

    •  Circuit Testing
    •  Connector Repairs
    •  Testing for Intermittent Conditions and Poor Connections
    •  Wiring Repairs

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

Control Module References for scan tool information

Circuit/System Testing
  1. Observe the scan tool DTC information. Verify there are no HO2S, or misfire DTCs set.
    2. ⇒If there are any HO2S or misfire DTCs set, diagnosis the applicable DTC before proceeding with this diagnostic procedure. Refer to Diagnostic Trouble Code (DTC) List - Vehicle .
  2. Inspect and verify the following:
    3. • The catalytic converter is an original part
    • The HO2S 2 is secure and the wiring is not damaged, or contacting the exhaust
    • The exhaust system for leaks, damage, or missing hardware
         ⇒If a condition is found, repair as necessary.

    Important: Before replacing the catalytic converter, correct any conditions that may have damaged the converter.

  3. If a condition is not located, replace the catalytic converter.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Catalytic Converter Replacement