P0420 Code Toyota: What Are the Causes and How To Fix Them?

The P0420 code Toyota is an error code that helps identify the problem with your vehicle so you can find ways to solve it. Therefore, we’ll discuss the meaning of the engine code, potential causes of the problem and recommend solutions to them.

P0420 Code Toyota

You’ll also discover the dangers of ignoring the trouble code and whether you can still drive with the problem. Finally, we’ll answer some popular inquiries regarding the P0420 code, so read until the end!

What Are the Causes of the P0420 Code Toyota?

The common causes of the P0420 code on a Toyota are a worn or old catalytic converter, antifreeze or oil seeping into the exhaust system, leaks in the exhaust system, and faulty spark plugs. This code on a Toyota means there’s an issue with your car’s catalytic converter.

Other causes include unburned fuel in the exhaust, a faulty oxygen sensor, and bad roads. The common symptoms of a damaged Toyota catalytic converter include a smell of rotten eggs coming from the back of the car, bad fuel economy and reduced engine power. Once you start noticing these symptoms, use an OBD II Scanner to confirm your suspicions then proceed to fix the converter.

– An Old or Worn Catalytic Converter May Not Function Well

Catalytic converters can last for up to 100,000 miles or around 10 years, after which they’ll start to malfunction. Some mechanics even advise drivers to examine their cat converters every 50,000 miles to ensure they’re working properly.

The converters’ main job is to turn dangerous exhaust gasses into harmless ones. However, a worn-out or old converter may not be able to do that; thus, it’ll throw up a P0420 code whenever the vehicle undergoes scanning.

Though the converters of Toyota vehicles are designed to last long, their lifespan can be shortened by most of the factors that are listed above. Thus, ensure you take care of it by avoiding bad terrain, but if you need to use a bad road, then drive with care.

– Oil or Antifreeze Seeping Into the Exhaust System

Engine oil or antifreeze leaking into the exhaust system can throw up a P0420 trouble code on your Toyota vehicle. Engine oil thoroughly lubricates the moving parts of the car engine, while the antifreeze ensures that the engine doesn’t overheat. Both have specific paths and seals that keep them from leaking into other parts of the vehicle. However, if the seals are broken or there is a leakage along their respective paths, these liquids will leak.

Oil is likely to cause fire when it gets into the exhaust system, especially the exhaust manifold, which is the hottest part of the system. In most cases, the oil becomes smoke first before bursting into flames.

One way to know that there’s antifreeze in your exhaust is by looking at the color of fluid coming out of the exhaust pipe. If the color is the same as that of your coolant, then it means that you have an exhaust leak. Another sign is that the smoke from the exhaust tends to be milky or cloudy after the gases mix with the antifreeze.

– Faulty Spark Plugs Missing Their Timing

Faulty spark plugs can cause misfiring in the engine when they miss their timing. They are responsible for igniting the car’s air/fuel mixture in the combustion chamber to produce power. Each of the components (including the spark plugs) involved in the combustion process is programmed to work at a specific “time” to ensure the process is successful. When the spark plugs are faulty, they may miss their timing (misfire) and allow some fuel to escape combustion.

Causes of the P0420 Code Toyota

This fuel may go through the engine and end up in the exhaust system unburned. When this happens, the fuel may ignite in the exhaust pipe due to the extremely high temperature. As a result, the engine will backfire and may damage the exhaust intake, leading to poor fuel economy. Also, the engine may backfire and won’t be able to produce enough power to keep your Toyota running.

– A Broken Oxygen Sensor Sending Wrong Signals

The oxygen sensor sends signals to the electronic control module on the amount of air to allow into the engine for combustion. If the sensor is damaged, it may send wrong signals that could lead to too much or too little air, which would affect combustion.

Also, the electronic control module would use the data from the damaged oxygen sensor to increase the amount of fuel for combustion. This then affects the catalytic converter as the extra carbons from the fuel will clog it, leading to overheating and subsequent damage.

– Leaks or Cracks in the Exhaust System

When there are cracks or holes in the exhaust system, pollutants can make their way into the catalytic converter and damage it. We’ve already come across some pollutants, such as antifreeze and fuel, which can cause engine backfire. Also, a leaking exhaust will draw more oxygen, resulting in lean combustion and affecting the performance of the vehicle. In some cases, the catalytic converter becomes clogged, allowing no exhaust fumes to pass through.

These fumes can end up back in the engine, which could cause a disaster. Worse still, there won’t be any air coming into the engine since the exhaust fumes can’t get out, which will render the engine dead. The exhaust manifold is especially dangerous when it leaks because not only will it cause the cat converter to fail, it can damage the oxygen sensors as well.

What Are the Practical Ways To Fix a Toyota’s P0420 Code?

Some practical ways to fix a Toyota’s P0420 code are to replace the old or worn-out catalytic converter, clean it, seal all the leakages, and repair the oxygen sensors. Then you’ll have to check which other components were damaged and repair them as well.

– Clean the Catalytic Converter to Fix the P0420 Code

There are several ways to clean a catalytic converter, and one such method involves adding a cat converter cleaner to the fuel. However, this method won’t work if the honeycomb is damaged or the exhaust system is clogged. Also, you’ll need to test if this method will work by tapping your converter with a wrench. If you hear noises, then don’t use this method, but if there’s no noise, then you can proceed.

Add the converter cleaner to the fuel in the right quantities (you can find the measurements on the label of the cleaner). Then drive the vehicle around to allow the cleaner to circulate through the exhaust system. Ensure you drive at 3,000 RPMs to get the converter hot enough to burn away the hydrocarbons in the exhaust fumes. As the cleaner works, you’ll notice less exhaust smoke and a decrease in the symptoms of a failed catalytic converter.

Another method involves removing the catalytic converter and cleaning it with a low-pressure washer and a degreaser. This method involves removing a lot of screws and the oxygen sensor as well. It can be quite complicated for a beginner but an experienced driver/mechanic will find it simple. We suggest you allow a mechanic to do the cleaning for you since the converter contains some delicate parts.

– Clean the Oxygen Sensors To Get Rid of Any Debris

To clean the oxygen sensors, you need to remove them, which can be a difficult task. However, you can make it easy by spraying a lubricant such as WD-40 on the sensors. Allow the lubricant to sit for 10 to 15 minutes, after which you unscrew the oxygen sensors with a wrench. Once the sensors are off, soak them overnight in a bucket or any container of gasoline and cover them.

Fix a Toyota’s P0420 Code

The gasoline will cleanse all the dirt and debris causing your P0420 Toyota symptoms. When morning comes, remove the sensors and dry them in the sun for an hour tops. Put the sensors back into their housing and screw them in place. Now, start the car a take it for a ride while keeping your eyes on the check engine light to see if it’ll illuminate.

– Replace All Faulty Spark Plugs To Stop Engine Misfires

First, remove all ignition cables and coins then remove the spark plugs from the engine with a wrench. If you encounter difficulty when removing them, you can either use a torque wrench or apply a lubricant.

Once the spark plugs are out, replace them with new ones and screw them back into place with a torque wrench. Remember, there’s no better way to fix a faulty spark plug than to replace it.

– Fixing Leaks or Cracks in the Exhaust System

One way to fix a leak in the exhaust system is to change the gaskets at the joints in the exhaust system. This means you’ll need to check all the gaskets for the troubled one and replace it. However, when the leak is not from the gasket but from a joint (flexible joint with a donut gasket), you’ll need to weld the area.

If you don’t know how to weld or don’t have the welding equipment, you can use epoxy bonds and tapes, but not without cleaning the area first. You can also use an aluminum patch to reinforce the epoxy bonds and wrap exhaust tape around the area. However, these are just temporary measures, thus, contact a mechanic for a lasting solution or replace the entire exhaust system.

Frequently Asked Questions

– Can Faulty Fuel Injectors Cause Code P0420 on Toyota Tacoma?

Yes, faulty fuel injectors can cause code P0420 on Toyota Tacoma when they fail to inject the right quantity of gas for combustion. This can lead to too much fuel, which leaks into the exhaust system and eventually into the cat converter and causes the engine to backfire.

– Can a Toyota P0420 Code Go Away After Fixing It?

Yes, a Toyota P0420 code can go away if you fix it properly or have a qualified mechanic make a proper diagnosis and fix it. If any part of the exhaust system or catalytic converter is broken, you can implore a welder to fix them.

Car Catalytic Converter

Can a P0420 Code in a Toyota Cause the Car to Run Fine?

When faced with a P0420 code in a Toyota, it is possible for the car to continue running smoothly. However, it is crucial not to ignore this code as it indicates a malfunctioning catalytic converter. Regular maintenance, following car troubleshooting tips, will ensure the issue gets addressed promptly and prevent any potential damage to the vehicle’s performance and emission control systems.


When you see a P0420 code on your Toyota, it’s an indication that your vehicle’s cat converter isn’t performing its function well.

Here are the possible problems and solutions:

  • The oxygen sensors are faulty, allowing either too much or too little air into the engine for combustion.
  • The solution lies in replacing these malfunctioning sensors with new ones.
  • Another cause is leaks in the exhaust system, which you can solve by either welding the cracks or wrapping them with epoxy bonds and tapes.
  • You can also clean a clogged converter by using a fuel additive or a low-pressure washer to clear all the filth.
  • A P0420 code on your Toyota Yaris isn’t serious but shouldn’t be ignored either because letting it linger for some time will harm the engine.

However, if the P0420 code keeps coming back, then we recommend you visit a Toyota dealership. You’ll likely find an expert who’ll suggest a perfect Toyota P0420 fix for your car.

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