P0420 Code: Everything You Need To Know

One of the most typical error codes is P0420. P0420 is a well-known code in the automobile sector primarily due to how annoying it can be to resolve. Fortunately, this article will assist you in understanding this error code’s meaning, potential causes, and solutions.

Experience has taught us that P0420 can be difficult to remove without the proper information, and one of the primary reasons for this is that if a part is replaced. The codes are reset; it may take a long time before the problem arises.

What Does The P0420 Code Mean?

The ECM has discovered underperformance of the catalytic converter’s efficiency on Bank 1 as indicated by the P0420 code. It can be a false warning from a malfunctioning O2 sensor or a defective catalyst. Two O2 sensors, one in front and one behind the catalytic converter, are used by the engine control module (ECM) to gauge its effectiveness.

The P0420 code will be shown if the catalyst system efficiency is low. A damaged catalytic converter is frequently the root cause of a P0420 fault code. A P0420 code can also be generated if an improper air/fuel mixture temporarily or permanently decreases the catalyst’s efficiency.

What Causes A P0420 Code?

A malfunctioning catalytic converter is the P0420 code’s most frequent cause. Moreover, a bad oxygen sensor, a leak in the intake or exhaust system, or an improper positioning of the catalytic converter could be to blame.

Close up old catalytic converter.

The catalyst may be at fault, but I’ve also seen plenty of instances when the trigger is brand-new but not an authentic factory component. Some less expensive catalysts might need to be more effective, so you need to get a new motivation from the manufacturer.

I’ve also seen non-OEM catalytic converters mounted too far from the engine on the exhaust pipe leading to the final muffler. As a result, the catalytic converter won’t get hot enough, and the P0420 error code will appear.

The catalytic converter might not work effectively if the car’s air-fuel mixture is off owing to a faulty injector, excessive fuel pressure, a malfunctioning fuel system, or a damaged sensor.

The following are the major causes of the P0420 code:

  • Catalytic converter damage (most common)
  • In the catalytic converter, there is oil pollution
  • Incorrect catalytic converter location
  • Faulty wirings or a damaged oxygen sensor, either upstream or downstream
  • Leak in the exhaust manifold or system
  • Wrong kind of fuel (leaded fuel instead of unleaded)
  • A blend of rich and lean resources (damaging the catalytic converter)
  • Misfires (destroying the catalytic converter) (damaging the catalytic converter)
  • Engine control module malfunction (rare)

How To Fix The P0420 Code

Before changing any parts, you should use the techniques described below in the article to test the catalytic converter’s functionality.

Making a good diagnosis is preferable to replacing parts without one, as doing so would most likely result in a waste of money. The following suggestions could resolve the P0420 problem code:

Oxygen sensor O2 in the exhaust pipe.
  • Catalytic converter cleaning
  • Catalytic converter replacement
  • Changing the front or back oxygen sensor
  • Achieve lower oil usage
  • Repair errors
  • Correct a rich or lean fuel mixture.
  • Engine control unit replacement (rare)

Common Symptoms of A P0420 Trouble Code

The dashboard check engine light is the most typical sign of code P0420. Rarely, you might experience problems like misfires, increased fuel use, or an unpleasant exhaust odor. But, besides the check engine light with the P0420 code saved in the engine control or powertrain control module, you won’t often notice any symptoms (PCM).

Common P0420 Code Causes by Car Model

Some automotive models have a higher prevalence of the P0420 error code than others. This diagnostic issue code is known to be problematic for some car models.

The most typical causes of each type of car are listed below. Remember that these are merely suggestions, and you should conduct a diagnosis before changing any components.

1.     Volkswagen (VW) / Skoda / Seat / Audi A4 1.8T / V6 2.4

On many VAG engines, a Check Valve/PCV Valve is under the intake manifold. You should check that first if you notice this issue code. To prevent the catalytic converter from becoming blocked, you must also ensure that the crankcase ventilation is sludge-free. A clogged crankcase ventilation system makes the engine burn oil.

Search for any engine codes associated with the oxygen sensors and any exhaust leaks around any flex pipes on the exhaust pipe, which are common causes.

If no issues were discovered, the catalytic converter should be changed. It is a common issue for both the 1.8T and the V6 petrol engines.

2.     Chevrolet

An exhaust leak frequently causes a P0420 code in your Chevy before the catalyst. Moreover, a malfunctioning catalytic converter or downstream oxygen sensor (rear) may be to blame.

View of Chevrolet car dashboard screen.

If you hear odd blowing noises when your engine is running, check the area surrounding the exhaust manifold because Chevy engines are known to leak there. P0420 recalls for GM cars are also available.

3.     Toyota & Nissan

A damaged catalytic converter is the most frequent reason for P0420 in Toyota and Nissan vehicles. Also, make sure no motor oil is burning in your engine. First, look for vacuum and exhaust leaks. Then, observe the exhaust pipe for any evidence of blue smoke.

If you notice blue smoke coming from the exhaust pipe, you may have a problem with crankcase ventilation, which might be as serious as a damaged turbocharger or worn piston rings.

Your catalytic converter is worn out if you don’t see any blue smoke at RPM. Nissan vehicles might experience similar problems.

4.     Ford

Ford engines frequently have vacuum leaks, broken EGR solenoids, or other defective parts, which result in an improper air-fuel mixture and the issue code. Use a diagnostic scanner to search the code memory for air-fuel mixture-related error codes.

Check for exhaust leaks if everything appears to be in order. If you cannot locate any error codes or other air/fuel combination issues, diagnose the catalytic converter and replace it.

5.     Subaru & Honda

The catalytic converter is probably to blame if your Subaru or Honda displays the P0420 code. A faulty oxygen sensor or an exhaust leak is a potential cause. Look for any engine codes relating to the fuel mixture or vacuum leaks. Inspect the exhaust system for any leaks before the catalytic converter.

The catalytic converter, however, is the Subaru engine issue that arises the most frequently. These boxer engines deteriorate quickly. It would help if you also looked at some of the P0420 recalls for Acura since they are very comparable to Honda.


The P0420 error code denotes that the engine control module has detected an issue with the catalytic converter’s effectiveness. A damaged, inefficient catalytic converter or a false alert set off by a sensor like the downstream or upstream oxygen sensor are also potential causes of code P0420.

Internal oil leaks that cause the catalyst to clog up and lose efficiency are another likely culprit.

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About Matthew Webb

Hi, I am Matthew! I am a dedicated car nerd! During the day, I am a journalist, at night I enjoy working on my 2 project cars. I have been a car nerd all my life, and am excited to share my knowledge with you!