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Discussion Starter #1
I’ve had this code in the past, cleared it and a month later it was back. Followed some steps in here about checking the temp on the different sensors and determined that in a regen it wasn’t getting hot enough. Replaced the 9th injector did a manual regen and code stayed away for a couple weeks. Now it’s back again.

Premium Member
155 Posts
Have you ever herd of anyone having the code intermittently pop up, on a EGR/DPF "weight reduction" vehicle? I have a buddy that is ongoing/currently having this issue. I'm guessing it's the tuning? Any input would be greatly appreciated

1,757 Posts
DTC P0420

Diagnostic Instructions

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.
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.
  1. Verify that exhaust leaks do not exist before the DOC. Refer to Exhaust Leakage. See: Engine, Cooling and Exhaust\Exhaust System\Testing and Inspection\Symptom Related Diagnostic Procedures\Exhaust Leakage
Circuit/System Testing

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:
  • 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
  1. Inspect the DOC for the following conditions:
  • 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.
  1. If all exhaust system inspections are normal, replace the DOC.
Repair Instructions

  • Catalytic Converter Replacement (4.3L) Catalytic Converter Replacement (4.8L, 5.3L, 6.0L, 6.2L) Catalytic Converter Replacement (6.6L)
  • Exhaust Particulate Filter Cleaning (LMM DPF Regeneration Enable) Exhaust Particulate Filter Cleaning (LMM DPF Service Regeneration) for Service Regeneration
Repair Verification

Refer to Diesel Particulate Filter (DPF) Service Regeneration Notice.

Certain engine control conditions can cause the EPF to overload with soot. It will be necessary to clean the EPF as instructed to remove the excess soot accumulation. To avoid the customer's return to the service department, perform the Exhaust Particulate Cleaning procedure
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