Audi A4 P0420: Catalyst System Efficiency → Below Threshold → Bank 1


Audi A4 P0420 Diagnosis


This page was created to assist you in troubleshooting the problem. Audi A4 P0420 trouble code. This covers the meaning of P0420, possible causes and solutions. The A4, OBDII, P0420 can often be caused by an bad oxygen sensorThere are many other reasons. You could waste your time and expense by rushing to replace the O2 sensors.  The OBDII number P0420 is most popular.


Índice de Contenido
  1. Audi A4 P0420 OBDII Code Defined
  2. Audi A4 P0420 Code Symptoms
  3. Here are the Top Reasons for and Solutions to P0420 in Audi A4
    1. Oxygen Sensor
    2. Catalytic Converter
    3. The O2 Sensor Wiring
    4. Timing
    5. Check for Leakage
    6. Engine Temperature Sensor
  4. Conclusion

Audi A4 P0420 OBDII Code Defined

P0420 It is an OBDII trouble code. Technically, the code stands for:

Low Catalyst System Efficiency

At least two oxygen sensors are installed in your A4, one just before the converter and another after it. One of their many duties is to determine the level of emissions coming into and out the converter.

If the downstream and upstream oxygen sensors give similar readings, then the vehicle will soon turn on the service engine (P0420).

These readings shouldn’t match, since the catalytic converter is supposed to be scrubbing the exhaust. The software on your A4 detects that the catalytic conversion isn't doing its job when the numbers match.


Audi A4 P0420 Code Symptoms

P0420 Audi A4

A common reason for the Audi A4's P0420 error code is that one or more of your oxygen sensors has failed. In most cases, the code will trigger the oxygen sensor before the converter.

There aren’t usually any drivability issues associated with P0420. Most people will notice the light at the top of the engine.

These are some of the symptoms you should look out for if your code is P0420.

  • Fuel Efficiency- You may or not experience any problems depending on the extent of catalytic conversion blockage.
  • Perilous times for Power– It is a really strange feeling to drive a vehicle with a catalytic converter that is going out. Typically, they’ll idle ok and drive normal under a light load. Whenever the engine is put under a heavy load it’ll feel like it is running out of gas. It’s very similar to the symptoms of a bad fuel filter.
  • Service Engine Light– Often, this is the only symptom of the Audi A4 P0420 code.

Related: P0442 Audi A4


Here are the Top Reasons for and Solutions to P0420 in Audi A4

You can have the trouble code P0420 in your A4 for many reasons. Below are the top ones.

P0420 can often be caused by an oxygen sensor. Specifically it’ll usually be the O2 sensor(s) on the upstream side of the catalytic converter.  But, don’t be surprised if it’s not. Checking the exhaust for signs of leakage is a smart idea. It should be possible to detect an exhaust leak underneath the vehicle.

Unless you have an exhaust leak, you’ll probably need to test the O2 sensors and/or catalytic converters. Below are some good guidelines to assist you in this task.


Oxygen Sensor

When the readings of both the exhaust oxygen sensors have similar readings, the P0420 code is thrown. If the oxygen sensor fails, the code can be triggered. If the engine does not exhibit any of the decreased performance associated with a bad catalytic converter, it very well may be the oxygen sensor (if you haven’t noticed the performance loss yet).

How to test an O2 or Oxygen sensor (YouTube)

New Oxygen Sensor – Audi A4 (Amazon)


Catalytic Converter

A catalytic converter is responsible for scrubbing out as much pollution as possible from the A4’s exhaust. These can build up over time. Even though modern catalytic convertors are designed to last the lifespan of vehicles, it is possible that there may be an underlying problem.

In 15 Minutes, Test your Catalytic Converter (YouTube)


The O2 Sensor Wiring

The oxygen sensor wiring may become corroded over time. Because it's right beside the exhaust, oxygen sensor wiring can become extremely volatile. It is volatile because it is further from the ECM.

In less than an hour, test your wiring (2 Auto Pros)



The P0420 code can be registered if your A4 has a problem with timing or misfiring.

Check for Leakage

If there’s a noticeable exhaust leak coming from the vehicle it can change what the O2 sensors register enough to throw the P0420 code.

Engine Temperature Sensor

If the computer doesn’t know what the engine temp is it’ll keep the fuel mixture rich. The O2 sensor could be thrown if the mixture of fuel and oil is too rich.



It's your task to find the root cause of Audi A4 P0420. You can comment on the article or send a message to suggest improvements. We are grateful.

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