P0420 Audi Fault Code, Cause And How To Fix

P0420 Audi fault code is an error in the catalytic converter not working properly. All information on how to recognize and fix instructions will be provided in the article below. The detailed information provided by Eurocartool will be of great help to technicians and vehicle owners experiencing this error.


What is error P0420 audi?

Error code P0420 Audi is appearing to indicate a problem with the catalytic converter on the vehicle’s exhaust system. It points to a potential problem with the efficiency of the catalytic converter in Bank 1. The main purpose of the catalytic converter is to reduce harmful emissions by converting them to less hazardous substances. The meaning of the code P0420 is that the catalytic converter is not operating optimally in terms of efficiency in reducing emissions.

  • Fault Technical Description P0420: Catalyst System Efficiency Below Threshold (Bank 1)
  • Severity of the problem: The severity of the problem is moderate. If you continue to drive without correcting this error code it could result in internal damage to your engine.
  • Repair Required: To prevent further damage to your vehicle’s exhaust system and possible engine damage, it is imperative that you have this repaired as soon as possible.
  • Diagnosis: Audi A4 P0420 is usually caused by a faulty catalytic converter. However, it can also be caused by other factors, such as malfunctioning oxygen sensor, exhaust leak…. You can take your car to a repair shop or learn how to self-diagnose and troubleshoot by referring to our guide P0420.


Error Cause

P0420 Audi is a fairly common trouble code found on vehicles with OBDII integrated ports manufactured since 1996 in general and on Audi A4 vehicles in particular. Error P0420 occurs when the sensor detects that the catalytic converter is not functioning properly or there is a problem with the amount of gas.waste does not meet standard quality.

The purpose of a catalytic converter is to reduce emissions. When the vehicle is started, the O2 sensor is also turned on to check the percentage of gaseous chemicals that are released from the exhaust system. When the sensor for hydrocarbons, carbon monoxide and other harmful gases in the vehicle exhaust exceeds the permissible limit, it means that the vehicle is increasing pollutant emissions into the environment.


When the central controller receives this result, it shows that the converter is not operating as efficiently as according to the set parameters. The system will automatically identify the part as worn out and in need of replacement. This is one of the causes of error warning P0420. In addition, the reason that the PCM system thinks the catalytic converter is no longer working may be due to:

  • Leaded fuel is used where unleaded fuel is required (unlikely)
  • Oxygen/O2 sensor damaged or faulty
  • Downstream oxygen sensor (HO2S) wire damaged or incorrect connection
  • Engine coolant temperature sensor not working properly
  • Damaged or leaking exhaust / catalytic converter / silencer / exhaust
  • Defective or ineffective catalytic converter (probable)
  • The oxygen sensor in front and behind the transmitter is giving too similar results.
  • Fuel injector leak or high fuel pressure
  • Misfire cylinder
  • Oil contamination

Expression of error P0420 Audi

The cause of the P0420 error comes from the exhaust part, so it will be difficult for you to detect major problems on the car. For example, if the catalytic converter is not operating in order, the exhaust emissions are affected resulting in the feeling of reduced vehicle power. Therefore, the error symptoms you can pay attention to when the vehicle shows error P0420 include:

  • Check Engine light is on
  • The engine lacks power, the car can’t accelerate much
  • Reduced fuel economy
  • The smell of rotten eggs or the smell of sulfur in the exhaust

Instructions on how to fix error P0420 Audi

In general, fault P0420 does not have a serious effect on the driver, however, if not repaired in time, some vehicle parts will be seriously damaged after a period of use. Here are some repair suggestions from Eurocartool experts:

Method 1: Clean the catalytic converter
Step 1: Prepare the catalytic converter cleaning solution
Step 2: Pour into Gas Tank
Step 3: Start the engine and drive about 150m, or start the engine at 2500 RPMs for 30 minutes for the cleaning solution to work


Method 2: Wash the catalytic converter
Step 1: Disassemble the catalytic converter
Step 2: Prepare a bucket full of water, pour in detergent,
Step 3: Soak the part for about 10 hours in the solution to remove all the dirt inside.

Method 3: Modify the Oxygen Sensor
Step 1: Remove the Oxygen Sensor
Step 2: Repair the Component
Step 3: Refit the Component
Step 4: Reinstall the Oxygen Sensor


Or you can fix the P0420 Audi error by using OBD-II scanner tools (TabScan 6154+C, SVCI 6154A, VAS5054,…) and Odis Service diagnostic software to coding to clear the error. It should be noted that simply removing the code can only provide a temporary solution to the problem. To ensure a permanent fix, it is necessary to address the root cause of the problem.

In short, when the vehicle shows the error code P0420, it means that the engine control module has detected that the catalytic converter is not working properly. Hopefully through this article you can better understand the information about this error code as well as the effective fix that we have provided.

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