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P0420 OBD-II Error Code: Meaning, Causes, Symptoms, & Fix


Diagnosing your car with the Error code P0420 is not initially a major problem. But it is good enough to trigger a few lights on your instrument cluster.

This particular OBD-2 error is caused due to malfunctioning catalytic converters. And, in this article, we will learn all about it. From the causes for “Catalyst System Efficiency Below Threshold“ error to its symptoms and fixes.   

P0420 OBD-II Error Code: Meaning, Causes, Symptoms, & Fix-dae8 #1-

What Error Code P0420 Means?

The OBD-II error code P0420 stands for: “Catalyst System Efficiency Below Threshold

This is an error caused due to a faulty converter. Where the catalytic converter of your car is not able to work at the least required efficiency. 

A catalytic converter is a part of the exhaust system that purifies the exhaust smoke and converts it into carbon-di-oxide and water vapor. To do so, it uses expensive metals like platinum and rhodium which work as a catalyst to initiate a chemical reaction. The reaction happens inside the converter and converts harmful gases into normal smoke. Hence the name Catalytic converter.   

But when the converter fails to do its job properly, that is not being able to filter the exhaust to a certain level, the error P0420 is raised. 

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What causes the “Catalyst System Efficiency Below Threshold error”?

While the P0420 error is caused due to an underperforming converter, it can happen due to multiple reasons. For one, the catalytic converter itself has failed, and maybe it is time to get a new one. Generally, a converter can last up to 10 years, but it can fail abruptly due to clogging, engine misfires, or even physical damage. 

A failed catalytic converter causing the error P0420 is extremely common. However, there are chances the error is raised due to other reasons.  

Failed oxygen sensor:

Even rare, failed oxygen sensors are good enough to relay wrong information to the car computer. Since the oxygen sensor is responsible for the proper air and fuel mixture, bad reading can cause the fuel mixture to be either too rich or too thin. 

The unbalanced fuel mixture can either cause the converter to overheat resulting in a meltdown, or failing the converter entirely. Additional error codes like P0174, P0171, P0172, or P0175 are a clear indication of too rich/lean fuel mixture. 

Leak in Exhaust system:

A leak in the exhaust system is very common after a long time. Leaks in the exhaust pipe or the catalytic converter cause the harmful gas to escape without filtering. 

This is not only dangerous but it also causes the oxygen sensor to make wrong interpretations. Which further down results in a faulty converter, hence causing the error P0420.  

Engine Misfire:

Bad spark plugs can cause misfires. In a misfire, there is either zero or incomplete combustion leaving the fuel unburnt. The unburnt fuel slowly causes the converter to clog, ultimately failing it. Error codes like P0300, P0301, P0302, P0303, P0304, P0305, P0306, P0307, and P0308 are all misfire codes. 

Engine oil/coolant leak: 

Engine oil and coolant can also get into the converter causing it to fail. It creates a layer of carbon and soot, which slowly clogs the converter, hence reducing the efficiency. The clogging of the catalytic converter not only stops it from cleaning the exhaust smoke. But creates back pressure which ultimately harms the engine. 

How do I know I have a Catalyst System Efficiency Below Threshold error?

The error P0420 is only caused due to a malfunctioning catalytic converter. This happens when it is either degraded or slowly degrading due to clogging. In such a case, it not only affects the emission but also the engine and performance. 

Common symptoms to look for are:

  • Engine check light on
  • Bad smell of Sulphur (rotten egg)
  • Lack of power from the engine
  • Bad mileage 
  • Failed Emission test    

The faulty oxygen sensor affects the fuel level which can decrease the fuel economy by 20%. Engine Misfire results in less efficient fuel burn which results in a lack of power production.

How to fix Error Code P0420?

The error code P0420 is caused due to multiple reasons. To fix the error, finding the source of the problem is the quickest way to go. Before that, you must ensure that the P0420 is the only error you get. 

While degraded catalytic converters are the main reason for the error. Misfire, leak, and bad oxygen meter are also possible causes. In such cases, when diagnosing you would get multiple error codes including the P0420. Fixing those error codes before should be the top priority. 

Only when you have solved the error, should you move on to the error P0420. 

Inspect Catalytic converter:

Start by inspecting the Catalytic converter. Look for any type of leaks or physical damage. Inspect everything from the exhaust manifold to the gasket, the catalytic converter, and even the pipes connecting to it. If a leak is found, get it repaired, clear the code, drive a bit, and run the scan again. 

If there isn’t a leak, the chances are that the converter is worn out and needs replacing. If you are facing bad engine performance with reduced acceleration, high heat generation with dark exhaust smoke, you have a degraded converter. 

Replacing it with a new one should be able to solve the problem. Do remember not to go for the cheap as they won’t last long. Even though it is expensive it is better to buy a decent catalytic converter. Depending on your car, a brand new catalytic converter can cost $400-$2400

Inspect the O2 sensor: 

If the converter isn’t at fault the O2 sensor might be. You can check the functioning of the O2 sensor using a digital meter. When your car is running, if the O2 sensor has a voltage of 0.45V, everything is working properly. But if the voltage is jumping between 0.1V-0.9V, then it is faulty, given that the converter is fine. 

Replacing the oxygen sensor with a new one costs in-between $200-$300. Which should be enough to solve the problem. 


Before moving to the catalytic converter or the sensor, ensure that error is not raised due to faulty wiring or software issues. If you don’t understand the reason for the problem or have trouble figuring the source, it is highly recommended to consult a licensed mechanic to fix the problem.