What Does OBD-II Fault Code P0420 Mean?

OBD-II Code P0420 is defined as a Catalyst System Efficiency Below Threshold

P0420 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

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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, 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
  • If you call a dealer to inquire about the warranty, have your VIN code ready to be checked in the manufacturer database, if you just call to ask about what is covered, typically the dealership will only tell you the minimal coverage. You need to be proactive and ask a Service Writer, not a phone receptionist, to check your VIN code for the most up to date warranty information.

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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5 User Comments

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By , September 07, 2010
Where can I get the information on Audi extending their warranty on the CAT for a P0420 code. I called 3 dealerships and they only quote the 8yr or 80K miles. Can someone provide me documentation for the extension?
By , September 10, 2010
Usually the Emissions Warranty is only 3 years or 50,000 miles so 8 years and 80,000 miles is an extension of the usual warranty. A few of the VW and Audi Models have had their Catalytic Converter Warranties extended even beyond 8yr/80k. The extra long warranty is determined by many things in addition to the Year/Model criteria, such as Production date, Manufacturing plant, Engine Serial number, Production date etc.
By , September 14, 2010
I understand the criteria. Where would I be able to find out what Year?Model etc would be covered under the extension. The post mentioned Audi and Volkswagen extending their warranty to 10yrs/100,000 for vehicles specifically having a P0420 code. I am not able to verify this anywhere.
By , September 14, 2010
The extension of the warranty is not for every Audi vehicle and it does not apply in every state. Once you have a proven P0420 code, have a service writer check you VIN in the database. The manufacturers are being forced into this against their own will, so please don't expect them to be very accommodating, in fact it may take some arm twisting. The original warranty for any Emissions System problem was 3 years and/or 50,000 miles, so the 8year/80,000 is already an extension of that. I would call Audi customer service as well. What specific year, mileage, and engine Audi vehicle do you have and do you have a confirmed P0420 CAT code? I also need to know what state you live in and if your car is Federal ( 49 state ) or California Emissions Certified.
By , September 16, 2010
2001 audi a6 quattro avant vin WAULH64B81N007798 Vehicle was purchased 2 weeks ago with 97K on it and a certified garage diagnosted the vehicle with a P0420 and P441 3 days after we purchased it when the light came on. We only have a powertrain warranty on it. I called the Audi Customer Service Center and they were CLUELESS. Thats why I asked for further information. 2 dealers stated it didnt come up in any service bulletin. Your help would be greatly appreciated as a new converter cost over $1,000 installed and used dealership does not want to assist in the repairs. We live in Pennsylvania.