P0420 Diagnostic Trouble Code

Published: March 24, 2022
Last updated: March 29, 2023

P0420 is a common diagnostic trouble code (DTC) that indicates "Catalyst System Efficiency is Less Than the Threshold (Bank 1)". Numerous factors can cause the Check Engine Light to light up. However, if your OBD-II scanner detected a P0420 code, your catalytic converter may be malfunctioning. 

Without the proper knowledge, a P0420 code can be quite difficult to manage. One of the primary factors for this is that when a part is replaced and the codes are reset, it can take an extremely long time before it reappears.

What Does P0420 Code Mean?

A P0420 code suggests that the engine control module (ECM) has determined that the catalyst efficiency is less than the threshold. This could be due to a poor catalyst or a false alarm generated by a faulty O2 sensor. Additionally, an improper air-fuel mixture can affect the catalyst, resulting in the P0420 code.

The ECM measures efficiency using two O2 sensors, one on the front and one on the back of the catalytic converter. If the efficiency of the catalyst system is insufficient, the P0420 code will appear. The most common cause of a P0420 trouble code is a faulty catalytic converter.

If the sensors are not functioning properly, the catalytic converter is also likely not working. In that case, harmful emissions from the vehicle are released into the atmosphere. Additionally, a damaged catalytic converter can also cause problems with other vehicle components.

Symptoms of P0420 Code

Though there are frequently no visible symptoms of a catalyst system failure, such as drivability issues, there are four common markers of the P0420 code:

Engine Light Is On

Numerous factors can cause the check engine light to illuminate, and a P0420 code is one of them. A mechanic will need to read the code and determine the exact cause of the problem using a diagnostic tool.

Slower Vehicle Speed

If your speed cannot exceed 30-40 mph, this may cause problems on major roads and interstates with higher speed limits. However, you may not recognize this condition if you drive more carefully through residential areas.

Loss of Power After Warming Up Vehicle

The engine will run more slowly and may lack the necessary power for acceleration, leading to performance problems.

Sulfuric Odor From Exhaust Pipe

This sulfuric odor, which may also smell like rotten eggs, is caused by an insufficient amount of oxygen in the catalytic converter. This results in an excess of sulfur in the fuel tank, which contributes to the exhaust pipe’s odor.

Common Causes of P0420 Code

Numerous factors can result in the appearance of code P0420 during a diagnostic check, making it difficult to resolve. The most frequently encountered issue is a defective or failing catalytic converter. Additional causes include the following:

  • Faulty muffler
  • Faulty exhaust manifold
  • Faulty exhaust pipe
  • Misfire of the engine
  • Tainted catalytic converter oil
  • Incorrectly installed catalytic converter
  • Sensor for engine coolant temperature failure
  • Leaking fuel injector
  • Increased fuel pressure
  • Air-fuel ratios that are too rich or too lean
  • Issues with the front or rear oxygen sensors
  • Damaged oxygen sensor wiring
  • Using the wrong fuel

How To Repair a P0420 Code

Before making any necessary repairs, you'll need to make an accurate code diagnosis. When diagnosing and trying to repair diagnostic trouble codes, you should refer to your application's factory repair information. Typically, the catalyst must be replaced, and bolt-on catalysts for the majority of vehicles are available.

Frequently, the only solution is to change the catalytic converter. However, numerous other problems can arise as a result of a catalytic converter failure. You must fix these problems prior to setting up a new catalytic converter, or the damage will likely worsen.

Damage to your catalytic converter will continue to wreak havoc on other components of your vehicle. Ensure that the business performing the repair is well-known for conducting thorough diagnostic checks. Other possible fixes include:

  • Clean-up of catalytic converter
  • Replacement of catalytic converter
  • Replacement of front or rear oxygen sensor
  • Rectify faulty sensor wiring
  • Resolve oil burns
  • Fix misfires
  • Adjust the fuel mixture if it is too lean or too rich
  • Change the engine's control unit

Estimated Repair Costs of P0420 Code

When P0420 is correctly diagnosed, it may require one or more of the following services to resolve the underlying problem. These are national average costs that include both parts and labor. Your cost will vary depending on the region and vehicle type.

Parts/ServicesEstimated Price
Air fuel sensor$200 - $300
Replacement of oxygen sensor$275 - $500
Catalytic converter$400 - $2,400
Leak in exhaust system$100 - $200

Before replacing your O2 sensors or catalytic converter, ensure that all other potential causes of inefficiency in the catalyst system have been ruled out.

It is better to contact a licensed mechanic to make sure you're getting an accurate diagnosis and avoid spending money on unnecessary parts replacement. You can also check with your auto repair insurance to see if they can cover some of these services.


While the catalytic converter issue is being resolved, different codes besides the P0420 may appear. You should fix those problems simultaneously with the P0420 code. They may shed light on a larger, system-wide issue.

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Managing Editor Before joining Keep Driving, Elaine was an editor for an international internet technology consulting firm. She shows tremendous interest in writing about future technologies such as Artificial Intelligence and Machine Learnings.
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