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P0420: Meaning, Symptoms, Causes, Diagnostics, and Fixes

The OBDII diagnostic code P0420 is one of the most common seen by mechanics. Want to know what causes this error code and how to fix it? We’ll break it down in full detail for you here.

P0420 Code Definition

  • P0420 Code Definition (Generic): Catalyst system efficiency below threshold (bank 1)
  • P0420 Nissan Definition: Catalyst System Efficiency Below Threshold Bank 1
  • P0420 Toyota Definition: Catalyst System Efficiency Below Threshold Bank 1
  • P0420 Honda Definition: Catalyst System Efficiency Below Threshold (bank 1)
  • P0420 Ford Definition: Catalyst System Efficiency Below Threshold (bank 1)
  • P0420 BMW Definition: Catalytic Converter Conversion (bank 1)
  • P0420 Chevy Definition: Catalytic Converter Efficiency Below Threshold (bank 1)
  • P0420 Subaru Definition: Catalytic Converter Efficiency Below Threshold (bank 1)

What Does P0420 Mean?

This code relates generically to the powertrain. It is typically prompted by issues with the catalytic converter. This is the part of your engine system that breaks down pollutants, converting them to harmless compounds like carbon dioxide.

In terms of severity, the P0420 error code doesn’t mean you or your car are in immediate danger. It is something you should get fixed as soon as possible, though. A malfunctioning catalytic converter can cause damage to your engine if left too long.

What Are The Symptoms of A P0420 Code?

The error code P0420 doesn’t typically cause large, noticeable issues. There are a few symptoms to keep your eye out for, however:

  • Activation of the check engine light
  • Smell of sulfur or rotten eggs from the tail pipe
  • Reduced gas mileage/increased fuel consumption
  • Reduced engine performance after warming up
  • Sluggish acceleration or reduced maximum speed

What Are The Causes of P0420?

As we said above, P0420 is related to the entire powertrain. There are a variety of specific issues that it could be pointing toward. These include:

  • Failure of the catalytic converter. This is the most likely cause of a P0420 error code.
  • Faulty or damaged oxygen sensor. This could mean the wiring is faulty, or it was not connected properly. It could also mean there’s an issue with the sensor itself.
  • Damage to the muffler. This code may activate if your muffler is leaking, reducing the efficiency of the catalyst system.
  • Damage to the exhaust system. Either your exhaust pipes or your exhaust manifold may be damaged or leaking.
  • Oil and fuel leaks. Contamination by oil in the catalytic converter could prompt this error code. Similarly, if the fuel injector is leaking, this code may activate.
  • Incorrect fuel type. The code will activate if you put leaded fuel in an engine designed for unleaded fuel.
An OBD2 scanner can help you detect the p0420 code.
You can detect the p0420 code by using OBD2 scanners.

How Do you Diagnose The P0420 Code?

Correctly diagnosing a P0420 error code is simple. You’ll need two tools:

Once you have your tools, follow these steps:

  1. Before you turn on your car, visually inspect the exhaust system. Look for any damage or leaks. Check to make sure all gaskets are intact, as well as examining the exhaust pipes and exhaust manifold.
  2. Repair any leaks or damage found in step 1, then clear the codes. Test drive your car and scan it again. If the trouble codes return, continue with step 3.
  3. Turn on your vehicle and let it idle for about 15 minutes to get up to operating temperature.
  4. Test the downstream oxygen sensor using the digital multimeter. The voltage should be steady, around .45V. If it jumps between .1v and .9v, the catalytic converter needs to be replaced.

What Common Mistakes Should You Avoid While Diagnosing Code P0420?

The main mistake people make with this code is assuming they know the cause of the error and making repairs before the diagnostics are complete. Often, inexperienced mechanics will simply replace the oxygen sensor. This won’t help if the problem is actually with another component.

Thoroughly diagnose each component before making replacements or repairs to save yourself unnecessary expense. One great tip is to check the temperature of the exhaust. It should be about 100°F hotter after the converter than before it once the engine is warmed up. If it’s not, this tells you the catalytic converter is the problem.

Another common mistake is to ignore other diagnostic codes that come up. There are about a dozen codes paired with P0420. If one of these gives a false reading, it could mistakenly activate code P0420. Fix any other codes that come up first. The actual problem might be elsewhere in your engine.

How Should You Fix The Code P0420?

The right way to fix the P0420 code depends on exactly where in your system it originates. Once you’ve verified the P0420 code was not given in error, fixing it likely means repairing or replacing one of the following components:

  • Catalytic converter (the most common cause of this error code)
  • Oxygen sensor (front, rear, or both), or the wiring and connections for the oxygen sensor
  • Engine coolant temperature sensor
  • Exhaust pipes
  • Exhaust manifold
  • Muffler
  • Fuel injectors

Looking for visual damage is one way to identify the faulty component, but not every issue will be visible. Since this trouble code can point to so many components, a certified mechanic should examine your car before you replace anything.

As with any repair, clear the trouble codes right away. Take your car for a test drive then check the live data again to make sure you’ve fixed the whole problem.

Tips To Avoid P0420 In The Future

Proper maintenance of your exhaust system is the best way to avoid P0420 trouble codes (and the associated, sometimes costly, repairs). Here are some simple tricks to maintain the health of your catalytic converter.

  • Take your car for a hard drive. Nicknamed an “Italian tune-up” by mechanics, this may seem counterintuitive but actually fixes a host of car troubles. Most cars never reach their most efficient temperature, especially in city conditions. Accelerate faster than usual for a few miles about once a month. This can help burn off build-up in the catalytic converter, as well as the exhaust and intake.
  • Clean your catalytic converter. Rather than burning off deposits, you can wash them away. Remove your catalytic converter and soak it overnight in a mix of degreaser and hot water. Rinse it thoroughly and allow it to dry before reinstalling it.
  • Engine cleaning fuel additives. You can buy solutions designed to clean your engine at any auto parts store. Alternatively, switching to a higher-octane fuel can help keep your engine running clean.
How to fix the code P0420.

Read more: OBD2 Codes: Full List Meaning & Fix Guide

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Tim MillerFounderOBD Advisor

I’m Tim Miller from Denver, Colorado. I’m the founder of obdadvisor.com, an automotive blog about "Diagnostic Tools and Auto Repair". My fan page is facebook.com/autozikcom. I've been working as an automotive mechanic and blogger for over 10 years writing articles to share my experiences and expertise.

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