P0420: Catalyst System Efficiency Below Threshold

There is a whole slew of error codes that owners are supposed to know about these days. However, few vehicle owners understand these codes, which means that when one appears, it sends them frantically to their keyboards searching for the answers to questions like, “how do I fix trouble code P0420?” Thankfully, you don’t have to look any further because we here at AutoZone have you covered. We are going to explain what the error code means, its causes, warning signs and even potential solutions that you can try at home if you’re savvy enough.

What Does the Code P0420 Mean?

While seeing any error code light up your dash is nerve-wracking, it is especially frustrating when you don’t know what it means. Error code P0420 signifies that the catalyst system efficiency is below the expected threshold. This code is considered a generic powertrain code that applies to all makes and models of vehicles produced from 1996 to present.

This particular code likely refers to issues in the exhaust system and more specifically, problems involving the catalytic converter. A catalytic converter’s job is to reduce vehicle emissions. However, issues can occur with the sensors, which can lead to reduced efficiency and other exhaust system problems.

What Are the Causes of P0420?

As is the case with most error code messages, it can be hard to pinpoint the root cause. For example, while the P0420 error code signals an issue within the catalytic converter, it can also signify problems throughout the exhaust system, which is why the check engine light often accompanies this code. However, some issues are likely, such as a failed catalytic converter, while others are unlikely, such as using the wrong fuel. Therefore, to find the culprit, it is necessary to understand all the potential contributing factors, including the two mentioned above. Consequently, in addition to the possibilities above, this error code can occur due to many factors:

  • Damaged or failed oxygen sensor
  • Damage to the wiring or connectors of the oxygen sensors 
  • Leaking or damaged catalytic converter, exhaust manifold, muffler, or exhaust pipe 
  • Retarded spark timing 
  • Readings too similar in oxygen sensors 
  • Cylinder misfire
  • Oil contamination 
  • High fuel pressure 
  • Leaking fuel injector 
  • Malfunctioning engine coolant sensor 

Are There Any Warning Signs?

Unfortunately, with this particular issue, there are no significant warning signs. Even the savviest driver or experienced mechanic might not notice a problem until the error code is displayed. However, there are times where a damaged catalytic converter can result in a restricted exhaust system, and it is possible you might notice a feeling of reduced power during acceleration in this instance. Other than that, this particular error will likely go unnoticed until the MIL or malfunction indicator lamp is illuminated.

How Do You Fix the Issue?

To fix this error code, you have a few options. First, you can search for and repair any exhaust leaks at the catalytic converter, manifold, and pipes. Second, inspect the downstream heated oxygen sensor to ensure proper operation. Third, use a scope to examine the process of the oxygen sensors, and finally, if all else fails, replace the catalytic converter.

If you do need to replace the catalytic converter, then you are better off using a unit from the original manufacturer if possible. If not, then you will want to choose a high-quality option that is suited for use in multiple states.

The OBD-II scanner error code P0420 problem is frustrating, but the issues that result in such an error are often inconspicuous. Therefore, you have no choice but to wait for the light to come on and then play detective, but routine maintenance and upkeep should help avoid any significant issues.

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