DTC P0420 Catalyst System Low Efficiency Bank 1

Circuit Description

In order to maintain a reasonably low emissions of Hydrocarbons (HC), Carbon Monoxide (CO), and Oxides of Nitrogen (NOx), the engine controls system uses a three-way catalytic converter. The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gas. This reaction converts them into harmless water vapor and Carbon Dioxide. The catalyst also reduces NOx, converting it to Nitrogen. The PCM monitors this process using Bank 1 HO2S #2. Bank 1 HO2S #2, located in the exhaust stream past the three-way catalytic converter, produces an output signal which indicates the oxygen storage capacity of the catalyst. This determines the catalysts ability to convert exhaust emissions effectively. If the catalyst is functioning correctly, the Bank 1 HO2S #2 signal will be far less active than that produced by Bank 1 HO2S #1. If a condition exists which causes the PCM to detect excessive Bank 1 HO2S #2 activity outside of an acceptable range for an extended period of time, the PCM will set DTC P0420. This indicates that the three-way catalytic converters oxygen storage capacity is below a threshold considered acceptable.

Conditions for Running the DTC

Conditions for Setting the DTC

The PCM determines the oxygen storage capability of the catalytic converter has degraded below a calibrated threshold.

Action Taken When the DTC Sets

Conditions for Clearing the MIL/DTC

Test Description

The numbers below refer to the step numbers on the diagnostic table.

  1. Before replacing the Three Way Catalytic Converter, correct any conditions which may have damaged the catalyst.







Did you perform the Powertrain On-Board Diagnostic (OBD) System Check?


Go to Step 2

Go to A Powertrain On Board Diagnostic (OBD) System Check


Are any other DTCs set?


Go to the applicable DTC table

Go to Step 3


Inspect for the following conditions:

  • Verify that the three-way catalytic converter is an original equipment part.
  • Inspect the three-way catalytic converter for damage (dents, severe discoloration caused by excessive temperature, holes, etc.).
  • Ensure that the internal three-way catalytic converter rattle caused by damaged catalyst is not present.
  • Inspect the exhaust system for leaks, damage, or loose or missing hardware (area from converter to HO2S #2 primarily).
  • Ensure that the HO2S #1 and the HO2S #2 are secure and that the pigtails and wiring are not damaged or contacting the exhaust.

Did you find and correct the condition?


Go to Step 5

Go to Step 4


Replace the catalytic converter. Refer to Oxidation Catalytic Converter Replacement in Engine Exhaust. Is the action complete?


Go to Step 5



  1. Select the Diagnostic Trouble Code (DTC) option and the Clear DTC Information option using the scan tool.
  2. Idle the engine at the normal operating temperature.
  3. Select the Diagnostic Trouble Code (DTC) option and the Specific DTC option, then enter the DTC number using the scan tool.
  4. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text, if applicable.

Does the scan tool indicate that this test ran and passed?


Go to Step 6

Go to Step 2


Select the Capture Info option and the Review Info option using the scan tool. Does the scan tool display any DTCs that you have not diagnosed?


Go to the applicable DTC table

System OK