Are you getting the dreaded P0420 error code from your Toyota Corolla? This post discusses the reasons for the code, and the easiest way to fix this common problem.

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Diagnosing a Check Engine Light Caused by the P0420 Error Code

Use an ODB2 check engine light reader to read the error codes that are causing your check engine light (CEL) to illuminate. If you’re not sure how to do this, read this post on how to read engine codes.

Error code p0420 on a toyota corolla.

Error codes that start with P042-, refer to a problem with the catalytic converter (cat). The way the vehicle determines that the catalytic converter is working okay is by measuring the oxygen content of the exhaust both upstream and downstream of the catalytic converter. If the cat is working, the 2 readings should be different. If the cat is not working and not doing anything to the exhaust, then the 2 sensors will read the same thing – indicating a problem.

Technically, this is what the P0420 error refers to:

P0420: “Catalyst System Efficiency – Below Threshold”

One thing to note: if the P0420 error code shows up in addition to any other error codes, diagnose those errors first. It is common that if there is another problem with your vehicle, the P0420 error code will be tripped as well. Diagnose the P0420 code last.

If you have any of these other error codes along with P0420, consider checking these problems first:

How to Fix a P0420 Error Code on a Toyota Corolla

When fixing a P0420 error code, start with the easiest and least expensive solutions, before progressing to the more difficult and expensive solutions. This list is ordered roughly from easiest/most likely to most difficult/expensive.

The other thing to ask yourself is, “Self, can I live with this error and constant check engine light?” The P0420 is generally not a safety issue in terms of the car being unsafe. It relates more to the efficiency (or monitoring) of the exhaust recovery system.

Keep in mind that if you leave the P0420 error code, then you check engine light will constantly be lit which could potentially make it so that you miss any other error codes that may come up in addition to the P0420. Also, check the regulations in your state concerning illuminated check engine lights, as the vehicle may not pass inspection with the CEL lit.

These are the Most Common Reasons for a P0420 Code

If you are getting a P0420 error code (and only the P0420) repeatedly, then you likely have one of the following problems.

Reason 1: Gas Blend, Outside Temperature, or “Bad Gas”

One reason for the P0420 code can be when the gas stations switch over to winter blend gas. As you may know, gas formulations change between winter and summer in many regions, and this can cause the vehicle emissions to change. In addition, colder outside temperatures can cause gas to combust more or less efficiently, changing the makeup of the exhaust. “Bad Gas” (everyone’s favorite excuse for any vehicle problem) can also be a culprit. The good news is that if any of these reasons are causing the P0420 to trip the CEL, it will likely reset itself once the car reaches equilibrium again, usually after filling the tank with fresh gas and driving for about 200 miles.

Reason 2: Filling the Gas Tank with the Engine Running

When the engine is running, a slight vacuum is pulled on the gas tank. When the engine is left running when the gas cap is removed, the vacuum is lost and can cause the vehicle to run rich, which causes unusual readings from the o2 sensors in the exhaust system.

For this reason, avoid filling the gas tank with the engine running. Besides the idea that it might be dangerous, it can help to avoid the P0420 code. By the way, if your P0420 code is caused by this, it should reset itself after driving for about 100-200 miles after your next fill-up where the vehicle is shut down.

Reason 3: Loose or Bad Gas Cap

For the same reasons described above, a loose or bad gas cap (that cannot hold a seal) can also cause the gas tank to lose its vacuum. If you have reason to suspect your gas cap, follow this guide to replace it.


Reason 4: An Exhaust Leak

An exhaust leak can cause erroneous readings to either the upstream or downstream o2 sensor. Think of it this way: if the o2 sensor is expecting to read no oxygen in the exhaust at a certain point, and there is a hole in the exhaust pipe just upstream of that sensor, it will read read oxygen (due to that coming in through the hole) and potentially throw a P0420 code.

One common exhaust leak exists when the “exhaust donut” breaks down. This commonly happens especially in older vehicles. Read this post for fixing an exhaust leak and replacing a deteriorated exhaust donut.

corolla exhaust flange

Reason 5: Bad Oxygen (o2) Sensor

One thing to consider is that the P0420 is being triggered not by the catalytic converter efficiency, but rather because the readings from one of the o2 sensors are incorrect. In this case, you will want to verify that the upstream o2 sensor and downstream o2 sensor is working properly. Read this post for information on diagnosing and repairing a bad upstream o2 sensor.


Another thing to consider is that the sensor itself may be okay, but the wiring could be cracked, broken, or exposed to the elements. Check the wiring itself to verify that it is not broken.

Reason 6: Bad Catalytic Converter

The P0420 code refers specifically to a problem with the efficiency of the catalyst system, which is the catalytic converter. If you have exhausted all other possibilities, it is likely that the problem is a fouled or failed catalytic converter. Though the catalytic converter is designed to last for the life of the vehicle, certain conditions can cause it to foul or fail prematurely (such as a vehicle that eats oil). The price of a new one is actually not horribly expensive. Check the current price on Amazon for a catalytic converter.

Other Less Likely Problems

There are potentially some other problems that can cause a P0420 code that are a lot less likely than those discussed in Reasons 1 – 6. These would also likely have other error codes associated with them as well, but for purposes of completeness, these should also be mentioned. These would include:

  • A problem with the coolant temperature sensor
  • A leak in the exhaust manifold
  • Bad fuel injector
  • Engine misfires
  • A timing issue with the spark plugs

I hope you have found this article helpful. Thanks for visiting!

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