P0420 Code: What It Is and How To Fix It

The catalytic converter is used to reduce the emission of harmful exhaust gases. It converts the high hazardous gases into less hazardous gases and water. When the vehicle computer detects an issue with the catalytic converter efficiency, it triggers the P0420 code. This article describes the P0420 code causes, meaning, symptoms, and how to fix it.

P0420 Code Definition

The P0420 code stands for “Catalyst System Efficiency Below Threshold (Bank 1).”

What is the P0420 Code Mean?

The P0420 code indicates that the powertrain control module (PCM) of your vehicle detects that the bank 1 catalytic converter is not working efficiently.

The catalytic converter is a crucial component of a car’s exhaust system that converts unburned fuel and carbon monoxide (CO) from the combustion process into carbon dioxide (CO2) and water (H2O). It also transforms nitrogen oxides (NOx) into nitrogen.

P0420 code

Your car has two oxygen sensors placed near the catalytic converter, one upstream and the other downstream. These sensors measure the catalytic converter’s efficiency.

If the car engine is operating at an optimal temperature or in a closed loop, the upstream O2 sensor’s readings should fluctuate, indicating its effective operation. On the other hand, if the catalytic converter is working efficiently, the downstream O2 sensor’s readings should remain unchanged.

If the readings of both O2 sensors are identical, there could be an issue with the catalytic converter. The downstream O2 sensor’s voltage typically decreases and fluctuates compared to the upstream O2 sensor’s voltage due to the higher oxygen levels in the system.

When the catalytic converter is unable to remove pollutants from the exhaust efficiently, it results in the P0420 error code. Although a malfunctioning catalytic converter doesn’t pose a danger to the driver, it can overheat and damage engine components.

Causes of P0420 Code

The P0420 engine code is typically caused by a malfunctioning catalytic converter or the catalyst system’s insufficient performance on bank 1. A faulty oxygen sensor, exhaust system leak, intake leak, or incorrect placement of the catalytic converter can also cause it.

When the air-fuel mixture in a car’s engine is incorrect, it may cause the catalytic converter to malfunction. This could be due to various issues, such as a faulty injector, high fuel pressure, a damaged fuel system, or a malfunctioning sensor. These problems can affect the efficiency of the catalytic converter and prevent it from functioning correctly.

The following are the major causes of the P0420 code:

  • Faulty oxygen sensors: If the upstream or downstream O2 sensors are malfunction, they may not be able to provide the catalytic converter with the correct input data, leading to the P0420 error code.
  • Exhaust leaks: Leaks in the exhaust system can introduce additional oxygen into the system, resulting in an incorrect reading from the O2 sensors and triggering the P0420 code.
  • Faulty engine coolant temperature sensor: A damaged or faulty coolant temperature sensor may cause the failure of the catalytic converter and trigger the P0420.
  • Malfunctioning engine: A poorly running engine that frequently misfires or burns excessive oil can cause damage to the catalytic converter, leading to a P0420 error code.
  • Leaking fuel injector: A leaky fuel injector is one of the major causes of a faulty catalytic converter.
  • Poor fuel quality: Using low-quality fuel or fuel contaminated with impurities can clog the catalytic converter or damage its internal components, resulting in a P0420 error code.
  • Wiring issues: Faulty or damaged wiring in the O2 sensor circuits can prevent the sensors from communicating correctly, leading to the P0420 code.
  • Use of wrong fuel: Wrong fuel is a major cause of the P0420 code.

Symptoms of P0420 Code

  • Illuminated check engine light: The P0420 error code triggers the check engine light to turn on, indicating an issue with the catalytic converter.
  • Decreased engine performance: A malfunctioning catalytic converter can cause decreased engine performance, including reduced power, acceleration, and fuel efficiency.
  • Poor idling: A poorly functioning catalytic converter may cause the engine to idle roughly or stall.
  • Smell of sulfur or rotten eggs: A malfunctioning catalytic converter can cause the emission of a sulfurous or rotten egg smell from the exhaust system.
  • Failed emissions test: This code can cause the vehicle to fail emissions tests, indicating an issue with the catalytic converter’s performance.

Read More: P0017 Code Symptoms and Causes

How to Diagnose the P0420 Code?

Here are the steps to diagnose the P0420 code:

  • Scan the Code: First, connect an OBD-II scanner to your car’s diagnostic port and read the code. This will indicate that the issue is with the catalytic converter.
  • Monitor catalytic converter efficiency: Use a diagnostic tool to monitor the catalytic converter’s efficiency over a period of time. If the efficiency does not improve, it may need to be replaced.
  • Check the oxygen sensors: Test the upstream and downstream oxygen sensors to ensure they are functioning correctly. You can do this by using a digital multimeter to test the voltage signal. If the sensors are not functioning correctly, they may need to be replaced.
  • Inspect the catalytic converter: Visually inspect the catalytic converter for any physical damage, such as cracks or dents. If any damage is found, the converter needs to be replaced.
  • Check for exhaust leaks: Check the exhaust system for any leaks. If there are leaks, it can introduce additional oxygen into the system, causing the catalytic converter to malfunction.
  • Check the engine performance: Inspect the engine for any performance issues, such as misfires, burning excessive oil, or a poorly running engine. Any issues with the engine can cause the catalytic converter to malfunction.
  • Replace the catalytic converter: If all other tests pass, the catalytic converter likely needs to be replaced.

Common P0420 Code Diagnostic Mistakes

There are several common mistakes that can occur when diagnosing the P0420 code, including:

  1. Neglecting to check for other codes: Other diagnostic trouble codes may be present, which could affect the catalytic converter’s performance. Failing to check for these codes can result in misdiagnosis and a failure to address the underlying issue.
  2. Assuming that the catalytic converter is the only problem: While a faulty catalytic converter can trigger the P0420 code, it is important to address any other underlying issues that may have led to the catalytic converter’s failure. Neglecting to address these issues can lead to premature catalytic converter failure and continued issues with emissions.
  3. Focusing solely on the catalytic converter: While the P0420 code indicates an issue with the catalytic converter, it is important to check other components of the exhaust system, such as oxygen sensors, exhaust pipes, and mufflers. Ignoring these components can lead to incorrect diagnosis and repairs.
  4. Not using the correct diagnostic equipment: An OBD-II scanner is required to diagnose the P0420 code correctly. Using incorrect or outdated diagnostic equipment can lead to misdiagnosis and incorrect repairs.
  5. Neglecting to perform a thorough visual inspection: A visual inspection of the exhaust system can reveal physical damage, such as cracks or holes, that can lead to a catalytic converter failure. Failing to perform a thorough visual inspection can result in incorrect diagnosis and repairs.

What repairs can fix the P0420 Code?

  • Resetting the engine codes
  • Fixing exhaust leaks
  • Replacing the faulty oxygen sensors
  • Replacing the faulty catalytic converter
  • Cleaning the clogged catalytic converter
  • Fixing the muffler leaks
  • Fixing the engine misfiring.
  • Repairing the leaky fuel injectors
  • Fixing the lean or rich fuel mixture
  • Fixing the exhaust manifold leaks
  • Replacing broken exhaust pipes 
  • Fixing the damaged wiring to oxygen sensors

How much does P0420 Code repair cost? 

The repair cost of the P0420 code varies according to relevant part repair, vehicle model, and labor cost. To fix the P0420 trouble code, you need to fix one or more of the below given:

  • Front O2 Sensor Replacement: 140$ to 320$
  • Catalytic Converter Replacement: 480$ to 1610$
  • Rear O2 Sensor Replacement: 130$ to 320$

How serious is P0420 Engine Code? 

The P0420 engine code typically does not pose a significant safety risk while driving. However, the malfunctioning catalyst system on bank 1 may result in the emission of harmful pollutants. Additionally, the check engine light and P0420 error code may cause you to fail emissions inspections.

It is always safest to promptly diagnose and repair any engine problems, including P0420. We do not recommend delaying repairs for several weeks or months.

If your vehicle feels unsafe or exhibits other significant symptoms or engine codes, it is best to err on the side of caution. However, it is likely safe to continue driving with the P0420 error code in most cases but use your best judgment.

FAQ Section

What are the causes of the P0420 Chevrolet?

The P0420 code in Chevy vehicles is commonly caused by an exhaust leak before the catalytic converter, a malfunctioning downstream oxygen sensor (rear), or a faulty catalytic converter. In particular, Chevy engines are prone to leaks around the exhaust manifold, so it’s important to inspect this area if you hear any unusual blowing sounds while the engine is running.

What are the causes of Toyota & Nissan?

The P0420 code is most commonly caused by a faulty catalytic converter in Toyota and Nissan vehicles. To diagnose the issue, check for vacuum and exhaust leaks first and then look for any blue smoke coming from the exhaust pipe. Blue smoke may indicate a problem with the crankcase ventilation or more serious issues such as damaged turbochargers or faulty piston rings. The catalytic converter is likely worn out if there is no blue smoke at any RPM. Similar issues may be found in Nissan vehicles.

What could damage the catalytic converter?

The catalytic converter may damage due to one or more of the below-given causes:

  • Bad PCM
  • Misfires
  • Lean mixture
  • Oil consumption
  • Exhaust leak
  • Rich mixture
  • Intake leak

Can I remove any parts to get rid of the P0420 code?

Simply removing parts to fix the P0420 code is not recommended, as it may result in additional trouble codes or other issues. Although it’s possible to reprogram the engine control unit to eliminate the catalytic converter’s monitoring system, doing so is illegal in most states and countries. Additionally, removing the catalytic converter altogether is not a viable solution, as it will likely cause you to fail emissions tests and may lead to legal complications.

Can I drive with a P0420 code?

Yes, it is possible to continue driving your vehicle with a P0420 code. However, it is not recommended, as the code indicates that the catalytic converter is not functioning properly, which can lead to increased emissions and decreased fuel efficiency. Moreover, driving with a faulty catalytic converter may also cause damage to other engine components in some cases.

It is best to have the issue diagnosed and repaired as soon as possible to ensure optimal performance and efficiency of your vehicle.

How to clear the P0420 code?

Using an OBD-II scanner is necessary to clear the P0420 code. However, it’s important to keep in mind that simply clearing the code may only offer temporary relief. To permanently resolve the issue, it’s necessary to identify and address the root cause of the problem.

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