Catalytic converter or o2 sensor? How to tell which one failed?

Did your mechanic recently scan your car with a diagnostic tool and the P0420 error code came up? If yes, then you must be wondering what it means, what triggered it, and how it can be fixed. P0420.

Error code is very common, especially for vehicles that have cloaked 100K miles. The P0420 error code is a generic OBD-II trouble code that indicates damage to the catalytic converter or faulty o2 sensors.

This error code indicates that the catalyst system efficiency is below the threshold or the o2 sensor is damaged.

The catalytic converter

The catalytic converter breaks down harmful pollutants that are created by your car during the combustion cycle by converting them to less harmful gasses.

This means that if this crucial component is not functioning efficiently, then your car will release harmful pollutants to the environment which can land you in trouble with the authorities.

The most common reason for error code P0420 is catalytic converter malfunction. The catalytic converter uses expensive metals like platinum, palladium, and rhodium to filter out fumes thus reducing the emission released from the exhaust pipe.

This device is equipped with two oxygen sensors, one sensor on the front of the catalytic converter (upstream) and the other at the rear (downstream).

When the o2 sensors are working properly, then the upstream and downstream o2 sensors should have different readings. If they have the same reading, then it indicates that the catalytic converter is damaged.

The o2 (oxygen) sensor

The O2 sensor is part of the emissions system of your car that measures the proportion of oxygen in your engine. The car engine works by burning gasoline. In order to burn fuel, most vehicles need a ratio of 14 grams of oxygen for every gram of gas. The oxygen sensor plays a crucial role in keeping that balance in check.

O2 sensors are located on the passenger side of the car and are mounted directly near the catalytic converter. When O2 sensors are damaged, your vehicle may lose up to 40% of its fuel efficiency because it will consume a lot of gas.

When a vehicle engine has a lot of air, then the engine is lean. On the other hand, if the engine does not have enough air, it is said to be rich. Both scenarios are not good for your car.

For instance, when the engine is lean, the car will not accelerate well. It can also cause jerking. A rich engine, on the other hand, causes the engine to run hot and emit harmful gasses because of incomplete combustion. The o2 sensors monitor your emission to ensure that both of these two scenarios don’t occur.

How to correctly identify which device is at fault

Error code P0420 can occur if the O2 sensors or catalytic converter are damaged. So, the big question is how do you identify which device is at fault? The best way to determine which of these two devices is at fault is by checking which one is damaged.

For instance, if the catalytic converter is damaged, your car will not be able to break down harmful pollutants created during the combustion cycle.

This means that your car will no longer filter and reduce harmful emissions such as nitrogen oxide, hydrocarbon, and carbon monoxide.

Common symptoms of a bad catalytic converter include:

  • Dark exhaust smoke
  • Reduced acceleration
  • Engine RPM instability
  • Sluggish performance
  • The smell of rotten egg or sulfur from the exhaust
  • Failure to pass oxygen test
  • Excessive heat under the car.

If your car shows these symptoms, then chances are the error code P0420 is caused by the bad catalytic converter.

On the other hand, if the error code P0420 is caused by a faulty O2 sensor, then it will not be able to tell how much-unburned oxygen is present in the exhaust while exiting the engine. If the O2 sensors are not working, then they will not transmit crucial information to the engine computer to trigger it to adjust the air-to-fuel ratio accordingly.

Common signs of a bad O2 sensor

  • Low fuel efficiency
  • Higher emission
  • Overheated catalytic converter
  • Poor engine performance (rough idling)

If your car shows these symptoms, the chances are that the error code P0420 is caused by bad O2 sensors.

How to fix the P0420 error

The method used to fix the P0420 error depends on the cause. If the error is caused by a faulty catalytic converter, then the best solution is to replace it with a brand-new one.

However, if the catalytic converter is clogged, then the problem can be fixed by unclogging it. On the other hand, if the P0420 error is caused by damaged O2 sensors, then the best solution is to replace both of them.

Repairing faulty sensors is difficult, which makes replacement the best option.

Does the oxygen sensor affect the catalytic converter?

YES, a faulty oxygen sensor will affect the performance of the catalytic converter. If oxygen sensors are faulty, the engine computer will not be able to set the air-fuel ratio correctly, resulting in lower fuel economy, high emissions, and damage to other crucial components.

Damaged oxygen sensors also cause the catalytic converter to overheat thus affecting its performance. If you want to prolong the lifespan of your catalytic converter, you should replace the faulty oxygen sensors.

Does the O2 sensor fail before or after the catalytic converter?

The O2 sensor usually fails before the catalytic converter. When it fails, it piles a lot of pressure on the catalytic converter causing it to overheat and eventually fail.

What happens if I unplug my O2 sensor?

When you unplug your O2 sensors your engine computer will not be able to calculate the amount of fuel that needs to be injected at a particular time.

This means that your vehicle will inject the same amount of fuel every time, resulting in poor fuel efficiency. Your car’s ability to accelerate will also be affected.

Should I replace the O2 sensor as well when replacing the catalytic converter?

YES, when replacing a bad catalytic converter, it is recommended that you replace the O2 sensors as well. If not, if the O2 sensors get damaged, then the catalytic converter will also get damaged.

However, you don’t have to replace the catalytic converter when replacing O2 sensors because a damaged catalytic converter will not affect the performance of the O2 sensors.

Temporary fixes for a bad O2 sensor


If a scan shows that your car has a P0420 error, then the problem could be a result of damaged O2 sensors or a catalytic converter. The symptoms that your car is showing can help you identify the exact cause of the problem.

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