P0420 Code

Code P0420 is a common emissions code that denotes an issue with a vehicle’s catalytic converter. It’s usually accompanied by a rough-running engine and a few other symptoms that make driving unpleasant and inefficient, so you’ll want to address it properly to restore vehicle performance and prevent future issues.

Learn more about the P0420 code below, from proper diagnostic methods to helpful repair tips that address the root cause. 

What Is a P0420 Code?

A P0420 is a short code that translates to “Catalyst System Efficiency Below Threshold (Bank 1),” which means that there’s a problem with the catalytic converter wherein the oxygen levels present in the exhaust system are below the required threshold in Bank 1. Bank 1 is the side of the engine that contains the number one cylinder, which is the first cylinder in the firing order. Because fuel combustion relies on oxygen, running with an insufficient level can damage a vehicle’s engine and exhaust system over time. 

The catalytic converter is one of the main parts of the exhaust system. It reduces gases by splitting the particles and breaking down pollutants after they leave the engine but before they enter the air. As such, the catalytic converter plays a critical role in vehicle performance by:

  • Transforming harmful emissions into less detrimental gases.
  • Reducing airborne pollutants that impact the environment.
  • Improving a vehicle’s efficiency and overall health.

You can examine the catalytic converter’s performance on the catalyst monitor. The monitor displays the converter’s current effectiveness in breaking down pollutants and reducing emissions using oxygen. It checks both the upstream and downstream sensors to assess the oxygen levels entering and leaving the converter. 

When the catalytic converter is not performing well, the Power Control Module (PCM) displays fluctuating movements of the downstream and upstream oxygen sensor voltages. The P0420 code indicates an unexpected oxygen level and low catalyst efficiency. 


Why Does the P0420 Code Happen?

There are various reasons why the P0420 code occurs, though a faulty catalytic converter is the most common. Other potential causes include:


Impact of a P0420 Code on Vehicle Performance and Emissions

In most instances, you can still drive even with a P0420 code, though there may be some noticeable changes like the check engine light blinking and reduced engine power output. A rotten egg smell coming from the exhaust pipe is another common symptom. 

While engine fault code P0420 is not a serious, immediate threat to safe driving, it can lead to worsening problems with the vehicle’s performance. One of the consequences of the conditions behind the code is failing emissions inspections, as poor converter performance means that your engine is releasing more pollutants than it would under normal conditions. As with any engine issue, whether small or large, it’s important to diagnose the problem as early as possible.


Diagnosing the Cause of a P0420 Code

If you notice your vehicle running rough, accelerating slowly or smelling like sulfur, it’s time to check your catalytic converter for performance issues. Diagnose the P0420 code by following the steps below:

  1. Use a reliable OBD-II scanner to check the exact code shown by the PCM. 
  2. Establish the baseline for the test, including the freeze frame data upon code retrieval. 
  3. Determine if there are common catalytic converter issues present, specifically in the reading of the upstream and downstream oxygen levels.
  4. Perform visual inspections for other possible causes like faulty wires, fuel system problems or ignition concerns. 
  5. Test-drive the vehicle and look into the freeze frame data to assess if the back sensor is operating correctly.
  6. Update the PCM to the latest version. 


Fixing the P0420 Code

Clearing a code P0420 starts with identifying its cause. From there, addressing the issue with one of these measures should eliminate the code:

  • Replace the faulty catalytic converter
  • Address oxygen sensor problems
  • Fix exhaust leaks
  • Resolve engine misfires
  • Replace wiring or connectors for the oxygen sensor

If you’re planning to fix the P0420 code on your own, you can do so as long as you have the right tools. Be sure to diagnose the main problem before making any modifications. If you’re unsure about the diagnostic and repair processes, seek professional assistance.


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