GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Holden 🢒 2008 🢒 Epica 🢒 Body Systems 🢒 Fixed and Moveable Windows 🢒 DTC P0420 or P0430

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

DTC P0430: Catalyst System Low Efficiency Bank 2

Circuit/System 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 HO2S 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
    • DTCs P0030, P0031, P0032, P0036, P0037, P0038, P0050, P0051, P0052, P0056, P0057, P0058, P0101, P0102, P0103, P0107, P0108, P0117, P0118, P0121, P0122, P0123, P0131, P0132, P0133, P0134, P0135, P0137, P0138, P0140, P0141, P0151, P0152, P0153, P0154, P0155, P0157, P0158, P0160, P0161, P0171, P0172, P0174, P0175, P0221, P0222, P0223, P0300-P0306, P0443, P0458, P0459, P2101, P2119, P2122, P2123, P2127, P2128, P2138, P2297, and P2298 are not set.
    • The vehicle speed is between 60-120 km/h (37-75 mph).
    • The engine coolant temperature (ECT) is greater than 80°C (176°F).
    • The calculated TWC temperature is between 550-862°C (1,023-1,585°F) and steady.
    • The DTCs run continuously once the above conditions are met.

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

DTCs P0420 and P0430 are Type B DTCs.

Conditions for Clearing the DTC

DTCs P0420 and P0430 are Type B DTCs.

Diagnostic Aids

Inspect for the following conditions, which 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

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. Engine running, observe the DTC information with a scan tool. Verify there are no HO2S or misfire DTCs set.
    2. ⇒If any DTC is set, refer to Diagnostic Trouble Code (DTC) List - Vehicle to diagnosis the applicable DTC, before proceeding with this diagnostic procedure.
  2. Verify the following conditions do not exist with the appropriate catalytic converter:
    3. • Dents
    • A severe discoloration caused by excessive temperatures
    • Road damage
    • An internal rattle caused by damaged catalyst substrate
    • Restrictions
         ⇒If a condition is found, replace the appropriate catalytic converter.
  3. Verify the following conditions do not exist with the exhaust system:
    4. • Leaks
    • Physical damage
    • Loose or missing hardware
    • Properly torqued HO2S
         ⇒If a condition is found, repair the exhaust system.
  4. Verify the following conditions do not exist with the appropriate HO2S 2:
    5. • A grounded wiring harness
    • Damage
         ⇒If a condition is found, replace the appropriate HO2S 2.
  5. If no physical condition is detected and the HO2S 2 is as active as the HO2S 1 after operating the engine at 1,500 RPM for 1 minute and then returning to a stabilized idle, then replace the catalytic converter.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Catalytic Converter Replacement