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For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2009 🢒 Sierra Denali 🢒 Driver Information and Entertainment 🢒 Cellular, Entertainment, and Navigation 🢒 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 provide an overview of each diagnostic category.

DTC Descriptor

DTC P0420: Catalyst System Low Efficiency

Circuit/System Description

The diesel oxidation catalyst (DOC), main function is to control emissions of Non Methane Hydro Carbons (NMHC) and soluble organic fraction (SOF). The secondary function of the DOC is to assist with the diesel particulate filter (DPF) regeneration, by obtaining and sustaining the temperature necessary for regeneration. The engine control module (ECM) monitors the exhaust gas temperature (EGT) sensor 1 to determine if the operating temperature of the catalyst is achieved during a regeneration event.

When the ECM detects that the DOC is not reaching a pre-determined temperature during an active regeneration event this DTC fails.

Conditions for Running the DTC
    • The ECM must be commanding an active regeneration.
    • DTC P0420 runs once during an active regeneration event.

Conditions for Setting the DTC
    • The ECM determines that the DOC functionality has degraded below a calibrated threshold, during a regeneration event.
    • The condition exists for greater than 4.5 minutes.

Action Taken When the DTC Sets

DTC P0420 is a Type B DTC.

Conditions for Clearing the DTC

DTC P0420 is a Type B DTC.

Diagnostic Aids
    • The use of Off Road Red Dye Fuel or continuing use of non ultra low sulfur fuel may be a possible contributor to this DOC failure.
    • If EGT sensor 1, before the DPF, is slow to respond to exhaust gas temperature changes, or sticks in range, this DTC may set.

Reference Information

Schematic Reference

Engine Controls Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

Diesel Particulate Filter (DPF) System Description

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 Verification
  1. Verify that DTCs P003A, P0047, P0048, P0101, P0201-P0208, P02A1, P02A5, P02A9, P02AD, P02B1, P02B5, P02B9, P02E0, P02E7-P02E9, P029D, P0300-P0308, P0402, P046C, P0545, P0546, P2032, P2033 are not set.
    2. ⇒If any of the DTCs are set, refer to Diagnostic Trouble Code (DTC) List - Vehicle .
  2. Verify that exhaust leaks do not exist before the DOC. Refer to Exhaust Leakage .

Circuit/System Testing

    Important: Perform the Repair Verification after completing the Circuit/System Testing.

  1. Inspect for the following conditions, which may cause a diesel oxidation converter (DOC) to degrade:
    2. • Off Road Fuel or Red Dye Fuel
    • Low quality B-20 fuel
    • Fuel Containing high sulfur content
    • Engine oil or coolant consumption
    • Leaking fuel injectors
  2. Inspect the DOC for the following conditions:
    3. • Physical damage or alterations
    • Internal rattles caused by a damaged catalyst substrate
    • Exhaust leaks before or after the DOC.
    ⇒ If a condition is found, replace the DOC.
  3. If all exhaust system inspections are normal, replace the DOC.

Repair Instructions
    •  Catalytic Converter Replacement
    •  Diesel Particulate Filter (DPF) Service Regeneration

Repair Verification
  1. Install any components or connectors that have been removed or replaced during diagnosis.
  2. Perform any adjustment, programming, or setup procedures that are required when a component or module is removed or replaced.
  3. Clear the DTCs.
  4. Turn OFF the ignition for 60 seconds.
  5. Duplicate the Conditions for Running the DTC and use the freeze Frame/Failure Records, if applicable, in order to verify the DTC does not reset. If the DTC resets or another DTC is present, refer to the Diagnostic Trouble Code (DTC) List - Vehicle and perform the appropriate diagnostic procedure.
  6. To avoid the customer's return to the service department, perform the Diesel Particulate Filter (DPF) Service Regeneration .