Why P0420 Code Keeps Coming Back?

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So you have checked and repaired the catalytic converter to remove the P0420 code. But after a certain period, it is back! You wonder why the P0420 code keeps coming back and how to remove the code permanently.

Why P0420 Code Keeps Coming Back?

The P0420 code is a generic OBD-II trouble code that indicates a problem with the catalyst system efficiency below threshold on bank 1. This code is commonly triggered by a faulty catalytic converter. But it can also be caused by other issues such as:

Oxygen sensor problems:

The oxygen sensors are responsible for monitoring the amount of oxygen in the exhaust gases before and after the catalytic converter. If one of these sensors is faulty, it can send inaccurate readings to the engine control module (ECM), which can cause the ECM to set the P0420 code.


Exhaust leaks:

An exhaust leak can allow unburned fuel and other pollutants to escape into the atmosphere. This can also cause the ECM to set the P0420 code.

Engine misfires:

Engine misfires can cause excess fuel to be burned, which can damage the catalytic converter and cause the ECM to set the P0420 code.

If the P0420 code keeps coming back after you have replaced the catalytic converter and oxygen sensors, there may be another underlying issue that needs to be addressed. Some possible causes include:

  • A vacuum leak: A vacuum leak can cause the engine to run lean, which can damage the catalytic converter and cause the ECM to set the P0420 code.
  • A faulty intake manifold gasket: A faulty intake manifold gasket can allow unmetered air to enter the engine. Which can also cause the engine to run lean and damage the catalytic converter.
  • A faulty fuel injector: A faulty fuel injector can cause too much fuel to be injected into the engine. It can damage the catalytic converter and cause the ECM to set the P0420 code.

If you are not sure how to diagnose or repair the P0420 code, it is best to take your vehicle to a qualified mechanic. They will be able to properly diagnose the problem and recommend the best course of action.

How long can you drive with the P0420 code on?

You can drive long enough with the P0420 code on until you completely damage the catalytic system or O2 sensor or face major damage. Driving further will damage the car even more. That is why I will recommend you not to push it too far. Of course, if you see the code while driving on the highway or going somewhere important, you can skip it for now. But fix the problem as soon as possible.

How much does it cost to fix a P0420 code permanently?

The cost of fixing the P0420 code permanently varies with the car model and underlying problem. A mechanic can make the diagnosis for 50-150 dollars and quote you the exact cost ($10-$2000) required to solve the problem. Here is a small breakdown.

  • Catalytic converter – $400-$2400
  • Air fuel sensor – $200-$300
  • Oxygen sensor – $275-$500
  • Repair an exhaust leak $100-$200 (if welding required)

📟 Other trouble codes
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People Also Ask

Let’s check some FAQs.

Can I fix the P0420 code myself?

Yes, you can fix some of the problems of the P0420 code yourself, such as cleaning, repairing small leaks, and even changing the O2 sensor. But I will recommend you take help from a professional to avoid any damage in CAT.

Will the P0420 code clear itself?

If you fix all the problems, the code will be cleared within an hour. Sometimes, you need to reset the code after fixing the problem. If the code does not clear and keeps coming back, there might be other problems.

Why am I still getting a P0420 code after I replaced my catalytic converter?

The code may appear due to a faulty O2 sensor, exhaust or sensor system leak, engine misfire, and others. Replacing the CAT does not guarantee a permanent fix of the P0420 code. That is why you are getting the code.

Will spraying carb cleaner inside the catalyst eliminate P0420?

No, spraying the carb cleaner inside the CAT does not eliminate the P0420 code unless the problem is with the clogged CAT. There might be other problems, such as damaged CAT or exhaust, leakage, loose connection, and others that cannot be fixed with the carb cleaner.

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