Error code P0420: What does it mean & What to do?

The Engine Control Unit (ECU) in your car is responsible for monitoring the emission system sensors. It is designed to optimize performance and keep the exhaust system emissions within acceptable limits. The o2 sensors in the emission system measure the catalytic converter efficiency. If the efficiency is lower than it should be, the ECU will store OBD-II code P0420.

Using a car diagnostic software or obd2 scanner you may find this Obd2 code from ECU.

So, Let’s dive in the details for what does the code P0420 mean, what are the causes and how to fix them.

What does it mean by sending Error Code P0420

The definition of the OBD-II P0420 code is “Catalyst System Efficiency Below Threshold (Bank 1)”.

When the ECU stores this code, it means that the catalytic converter is not operating at maximum efficiency and may need replacement.

error code p0420
Car fault code: P0420

Taking a closer look at the catalytic converter, it looks a lot like the muffler in shape and size. It is made of platinum, palladium and possibly rhodium which are all rare metals. This is why the catalytic converter is so expensive to replace.

The rare metals used, reduce the toxicity of the exhaust gasses. The rare metals make it efficient but also susceptible to damage if the engine or other issues are not addressed. While operating, the converter utilizes two O2 sensors. One is located in front (upstream) while the other is located after (downstream) of the catalytic converter.

During normal operation, if those two O2 sensors indicate the same readings, the catalytic converter will not be operating within specifications. This will result in higher than normal oxygen levels. Thus, it will also signal the ECU to store the P0420 code and turn on the check engine light.

You probably won’t notice that there is a problem with the catalytic converter but the environment will definitely be impacted.

What are the Causes and Symptoms of OBD-II code P0420?

The catalytic converter depends on the oxygen sensors to operate efficiently. For that reason, it is important to perform an oxygen sensor test before replacing or cleaning the catalytic converter. To perform the oxygen sensor test, refer to the article “How to test a bad o2 sensor” for instructions.

P0420 Causes

A comprehensive list of other issues that could cause the ECU to store a P0420 code is as follows:

  • Muffler damage or leaks
  • Exhaust manifold damage or leaks
  • Exhaust pipe trouble or leaks
  • An engine misfire or skip
  • Oil leaking into the catalytic converter causing contamination
  • Defective engine coolant temperature sensor
  • Defective front or rear o2 sensors
  • Damaged o2 sensor wiring or connections
  • Fuel injector leak
  • Higher than normal fuel pressure

As you can see there are quite a few issues that could cause the P0420 code to be stored by the ECU. It is worth taking the time to check them out thoroughly before you replace the catalytic converter.

P0420 Error Symptoms

This P0420 obd2 code indicates that there is a problem with the catalytic converter, oxygen sensors or both. When this code is stored by the ECU, the malfunction indicator lamp will come on. This is your first sign that there is a problem somewhere in the emission system.

Other signs to watch for are lack of engine power, decreased fuel economy or a rotten egg (Sulphur) smell. If any of these other signs appear then you need to get the ECU scanned. This will tell you if there is one or multiple codes stored, that are contributing to the problem.

The symptoms of a bad catalytic converter  which may lead to P0420 fault code includes one or more of the

  •  Check engine light comes on
  • Lack of engine power when accelerating
  • A decrease in fuel economy
  • Rotten egg (Sulphur) smell
  • Engine misfires or skips
  • The car is hard to start

Now, let’s look a little deeper into how to diagnose and fix a code P0420.

Code P0420 Test/Diagnostic

As previously stated, there are quite a few issues that could cause the ECU to store P0420 code. So where do you start to try and figure out what the problem is?

The best way to start is to refer to a list of common issues and eliminate them one by one. By doing this you will eventually uncover the issue.

Let’s take a look at some of the things that you can do to diagnose the problem:

  1. Performing a visual check of the muffler, catalytic converter and exhaust pipes for any damage is the easiest place to start. If the catalytic converter has any dents, holes, discoloration or other damage, it probably needs to be replaced.
  2. Visually inspect the upstream and downstream o2 sensors for any loose connections or disconnected wires.
  3. Check to see if there are other codes paired along with the P0420. If so, check out those codes first as they can generate false readings and trigger the P0420 code.
  4. Determine if the engine is running rough or there is a noticeable skip.
  5. Check to see if the temperature sensor indicates a very high or very low engine temperature (bad sensor).
  6. Look at the fuel injectors to see if there is excess fuel leaking around one or more of them.
  7. Run the car until it reaches normal operating temperature (may take up to 20 minutes). Take the digital multimeter and check the voltage of the downstream o2 sensor. It will produce a relatively stable .045V if the catalytic converter is working properly. If the voltage is wavering between 0.1V and 0.9V then there is a problem with the catalytic converter, and it should be replaced.
  8. Another more sophisticated diagnostic involves reviewing the Mode 6 test results. Mode 6 tests are the actual self-tests that the ECU runs through to detect faults and monitoring readiness of the ECU. It is important that the front and rear o2 sensors pass the Mode 6 tests. If they fail, false readings could potentially confuse the OBD-II software and trigger a false P04020 code.

How to fix Fault code P0420

In most cases, the failing catalytic converter is not dangerous and poses no risk to the occupants of the car. The real risk is that serious damage to the catalytic converter may occur unless the P04020 code is resolved quickly.

Since the catalytic converter cost money to repair, it’s worth taking the time to fix the P04020 code as quickly as possible.

Recommended repairs to fix OBD2 code p0420

Based on the results of the diagnostics, here are the repairs that may need to fix code P04020:

  • Address and fix any other codes paired along with the P0420 first as they may be generating false readings.
  • Check and fix the muffler and exhaust pipes.
  • Repair/change the exhaust manifold.
  • Check and fix the engine coolant temperature sensor.
  • Test and replace one of the O2 sensor (or both).
  • Test the wiring or connections to the o2 sensor(s).
  • Identify and fix the problem causing an engine misfire.
  • Check and fix any leaking fuel injector.
  • Clean the catalytic converter using a cleaner that will remove any deposits or gunk in the system.


It is important to note that the ignition, fuel and air intake systems can contribute to damaging the catalytic converter. As a matter of fact, these systems are the most frequent causes of the P0420 error code. For this reason, we recommend that these systems are thoroughly checked out before replacing the catalytic converter.

So after performing all the diagnostics, analysis and repairs, the P0420 code still stores in the ECU. Unfortunately, this means that you will have to consider replacing the catalytic converter.

The good news is that you did everything possible to eliminate any doubt that the catalytic converter is bad. The bad news is that you have to choose between going to a professional or attempting to do it yourself.

No matter what you decide, you should replace the o2 sensors (recommended) and catalytic converter with high-quality OEM replacement parts.

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