What Does The P0420 Code Mean For The Car’s Engine?

It is common for a driver to experience no driveability problems when the P0420 trouble code is present. Other than the Check Engine Light being on, the symptoms of this trouble code may go unnoticed. However, if the vehicle is left in error without addressing the problem, serious damage could be caused to other components.

Because there are no driveability issue symptoms associated with the P0420 code, it is not considered serious or dangerous to the driver. However, if the code is not addressed promptly, the catalytic converter could be seriously damaged. Because the catalytic converter is expensive to repair, the P0420 trouble code must be diagnosed and repaired as soon as possible.

What Does The P0420 Code Mean?

Code P0420 stands for “Catalyst System Efficiency Below Threshold (Bank 1).” Put simply, this code indicates that your car’s computer has detected underperformance from the Bank 1 catalytic converter.

The purpose of the catalytic converter is to break down harmful pollutants that are created during the combustion cycle. By using fine platinum and gold meshes to filter exhaust fumes, the catalytic converter can reduce the emissions expelled from the exhaust pipe.

The catalytic converter has two oxygen sensors. One oxygen sensor is positioned in front or upstream of the catalytic converter and the other oxygen sensor is positioned at the rear or downstream.

If the upstream oxygen sensor is working properly, its readings should fluctuate. This occurs when the car is at operating temperature and when running in a closed loop. If the downstream oxygen sensor is working properly, and there isn’t an issue with the catalytic converter. But its readings should remain steady.

What Are The Possible Causes Of The P0420 Code?

A faulty catalytic converter is the most frequent cause of a P0420 trouble code. Issues that can cause this OBD-II code include:

  • A failed catalytic converter is very common.
  • A faulty oxygen sensor is not very common.
  • Circuit problems, such as damaged wiring and loose connections which are not very common.
  • An exhaust leak is not very common.
  • Computer issues e.g., software in need of an update which is not very common.

The catalytic converter can also fail due to several other problems. These include engine performance issues, such as a misfire or improper air to fuel ratio, which can cause the converter to overheat.

Symptoms Of The P0420 Code

Since the P0420 code is linked to the sensitive components, it is crucial for you as a driver to keep an eye on any symptoms. This indicates that the catalyst system efficiency is not at the proper level by monitoring symptoms of a PO420 code.

Luckily and as experts in the automobile field, we collected some of the common symptoms that you might look for to confirm that your vehicle suffers from a P0420 code, including:

  • Whenever you warm the vehicle up, you’ll still deal with power loss.
  • When you try to accelerate over 30 to 40 mph, it won’t happen because your vehicle might experience some slow acceleration.
  • You might smell some sulfurous smell coming from the exhaust system.
  • You will notice a significant reduction in the fuel economy.
  • Also, you might experience some issues with a reduction in drivability, rough idling, hard shifting, and sometimes misfiring.
  • Your vehicle will not have the required air to fuel mixture ratio.
  • You might see some blue smoke coming from the exhaust system as your vehicle might be burning oil.
  • Obviously, with a P0420 code, your vehicle will not be able to pass the emission test.

How to Diagnose Code P0420:

Whenever you face a problem with the P0420 Code always follow the below step to diagnosing the problem.

1. Scanning

Scan your vehicle to verify P0420 is the only code present. If other codes are present, they must address first.

2. Examination

Examine the exhaust system for damage and leaks. Pay close attention to the exhaust manifold, gaskets, and exhaust pipes pre-catalytic converter. If any leaks are found, repair the leak, clear the code, and complete several drive cycles to verify that is the fix.

3. Check The Voltage Reading

With the vehicle running and at normal operating temperature, use the digital multimeter to check the voltage reading of the downstream O2 sensor. The downstream O2 sensor produces a relatively steady voltage reading of approximately 0.45V in case the catalytic converter is functioning properly. In case the voltage of the downstream O2 sensor is constant between 0.1V and 0.9V, the catalyst is worn and the catalytic converter needs to replace.

Can a bad O2 sensor cause a P0420 code?

A bad O2 sensor can easily trigger the P0420 code here at. This sensor might wear out over time, requiring replacement. Your vehicle’s only way to tell you that the O2 sensor has a problem is by triggering a check engine warning light.

Whenever you see the check engine light, the problem is coming from issues with the oxygen sensor. Therefore, you need to use an OBDII scanner to read through any existing codes and make sure that 100% confident it’s coming from the oxygen sensor.

Estimated Cost of Repair

For error code P0420, one or more of the below repairs may need to solve the underlying issue. For each possible repair, the estimated cost of repair includes the cost of the relevant parts and the cost of labor required to repair.

Air fuel sensor or oxygen sensor 200$-300$

Catalytic converter 400$-2400$

A leak in exhaust 100$-200$ (if welded to repair)


When your vehicle’s check engine light illuminates, you must not ignore the problem, and you must take immediate action to see what is causing it. Sometimes the check engine light might illuminate due to a simple issue like a loose fuel cap. And also can link to complicated problems like complete engine failure.

The P0420 code is linked to a critical component which is the catalytic converter. This catalytic converter is very expensive. Whenever you see any indication showing that the catalytic converter suffers from something, you must immediately take care of the problem. Just to avoid very expensive repair costs.

If preparing the vehicle to clear the P0420 code will cost you a lot. We advise you to think and evaluate the situation about whether it’s worth selling your car or not.

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