P0420 - OBD-II Trouble Code

Auto Systems and Repair

Catalyst System Efficiency Below Threshold

Our emissions expert has put together the following information about the P0420 fault code. We have also included diagnostic procedures you can take to your repair shop if the mechanic is having difficulty analyzing the code.

OBD II Fault Code

  • OBD II P0420

Fault Code Definition

  • Catalyst System Efficiency Below Threshold

Symptoms

  • Check Engine Light will illuminate
  • In most cases, there are no adverse conditions noticed by the driver
  • In some cases, there may be some performance problems noticed by the driver such as a lack of power from a restricted and/or damaged Catalytic Converter

Common Problems That Trigger the P0420 Code

  • Inefficient Catalytic Converter(s)
  • Defective Front or Rear Oxygen Sensor(s)
  • Misfiring engines

Common Misdiagnoses

  • Oxygen Sensors

Polluting Gases Expelled

  • HCs (Hydrocarbons): Unburned droplets of raw fuel that smell, affect breathing, and contribute to smog
  • CO (Carbon Monoxide): Partially burned fuel that is an odorless and deadly poisonous gas
  • NOX (Oxides of Nitrogen): One of the two ingredients that, when exposed to sunlight, cause smog

Notes

  • Volkswagen , Audi , Dodge and Toyota have extended their catalytic converter coverage on many of their vehicles. To find out more about this information on RepairPal.com, enter P0420 followed by your vehicle make in the search bar above, for example, P0420 Audi.
  • Most Toyota vehicles need an OEM Catalyst, especially if there is a Catalyst-related software update required. Toyota vehicles usually re-set the P0420 code unless an OEM catalyst is used as a replacement. It is common for mechanics to replace all the Oxygen sensors only to find that the Toyota vehicle required an OEM Catalyst.
  • Most of the aftermarket Converter companies are re-working their designs so they can be certified for use in OBD-II vehicles in California

The Basics
The Catalytic Converter looks like a muffler. It is typically a stainless steel housing over a ceramic honeycomb core. The catalyst itself is made of platinum, palladium, or rhodium, all rare metals, which is why Catalytic Converters are so expensive. These elements reduce the toxicity of harmful exhaust gases that are expelled from the tail pipe. Catalytic converters are quite efficient, but if engine maintenance is neglected or an engine is allowed to "run rough," damage could occur, resulting in costly repairs. To replace the Catalytic Converter, the vehicle is raised to gain access to its underside. The converter is removed from the exhaust system and the new Catalytic Converter is installed.

Want to Learn More?
The Catalytic Converter is a sophisticated after-burning device designed to complete combustion of the exhaust gases that pass through it. It is a stainless steel container with an inlet and outlet pipe that looks similar to a muffler. Inside, the Catalytic Converter is a ceramic monolithic structure that has honeycomb-like passages running though it. This structure has several sections called beds that are thinly coated with rare metals, which react with the compounds in the exhaust gases to complete the combustion process, thereby cleaning the exhaust of harmful emissions.

  • The first section of the Catalytic Converter is called the reduction bed and is coated with rhodium. It is called the reduction bed because its purpose is to reduce the NOx gases back into harmless nitrogen and oxygen.
  • The next section of the Catalytic Converter is the oxygen storage bed, which is coated with cerium. Its purpose is to maintain an ideal level of oxygen for use by the rear of the converter. It does this by storing and releasing the oxygen that gets released from the reduction of NOx in the previous reduction bed.
  • The oxygen is then available for use in the final oxidizing bed, which is coated with platinum and palladium. The purpose of the oxidizing bed is to complete the combustion of CO by adding oxygen. The oxidizing bed also uses oxygen to burn any of the raw HCs that still remain in the exhaust gases.

P0420 Diagnostic Theory for Shops and Technicians
The P0420 code is set when the Catalyst monitor sees a decrease in voltage from the rear Monitoring Oxygen Sensor(s) and an increase in switching activity—from rich to lean to rich, etc.—that closely resembles the front Oxygen Sensor(s) during the time the computer is activating the Catalytic Converter monitor test. The voltage threshold is usually a minimum of 650 millivolts, which indicates a low level of oxygen. When the voltage goes too far below the 650 millivolt minimum, it indicates a higher level of oxygen. This means that not all of the oxygen is being consumed by the combustion process or by the afterburning effect of the Catalytic Converter. When the oxygen level gets too high, it means that the Cerium or Oxygen storage bed has degraded to the point where it is no longer able to store oxygen created by the reduction of NOx (nitrogen and oxygen). This oxygen is essential for the rear Oxidation bed to complete the conversion of CO into CO2 and HCs into H20 and CO2.

Common Tests for Diagnosis of the P0420 Code

  • Retrieve the code and write down the freeze frame information to be used as a baseline to test and verify any repair.
  • If there are any misfires, ignition, fuel and/or intake problems, these must be repaired before the Catalyst code is addressed. Any misfire, ignition, and/or fuel system problem will quickly ruin a Catalyst. They are often the cause for code in the first place.
  • Test drive the vehicle at or near the freeze frame conditions to verify that the rear Catalyst Monitoring Oxygen sensor is either mirroring the front Oxygen sensor and/or is not reaching the 650 millivolt threshold during 55–60 MPH cruise conditions. If either of theses conditions can be easily verified, then the Catalytic Converter is defective.
  • If there is any doubt about the condition of the front and/or rear Oxygen sensors, then check the Mode 6 data for all the Oxygen sensor monitor tests. If any of the front or rear Oxygen sensors barely pass their Mode 6 tests, then clear all codes and perform a drive cycle to see how well the front and rear Oxygen sensors pass their Mode 6 tests. They need to pass the Mode 6 tests with flying colors or they will confuse the OBD-II diagnostic software and possibly trigger a false code P0420. This is an important concept because if the front Oxygen sensor is slow and barely passes its monitor tests, it can fool the computer into thinking that the Catalyst has failed because the computer just watches how closely the switching speed of the front Oxygen sensor compares to the switching speed of the rear Monitoring Oxygen sensor. If the rear Monitoring Oxygen sensor is losing bandwidth and not able to easily reach the 650 millivolt threshold—but still barely passes its monitor tests—then it, too, can fool the computer into setting a P0420 code.
  • If the Catalyst has failed, be sure to check for any software updates for the powertrain computer. Many of the OBD II-equipped vehicles require software updates whenever the Catalyst is replaced.
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