P0420 Honda Accord: Everything You Need to Know

The worst news you can get when you run a code reader on your car is any type of code. And if you’re reading a P0420 code, it could mean a number of issues. If you’re the driver of a Honda Accord and you happen to see this code, what should you do? Let’s discuss.

General Information

Simply put, a P0420 Honda Accord code means that the catalyst system’s efficiency is below the threshold. In terms of severity, this is a moderate code. While nothing will happen immediately, extended driving once you notice this code could contribute to engine damage. As a general rule, you should get this problem repaired as soon as possible. You’ll prevent damage to your engine and emission system.

Besides the reader showing the error code, you might also notice your “Check Engine” light is lit up. As you’re driving the car, you may experience a decrease in power. Issues that cause a P0420 error might also harm your fuel economy. It’s also possible you’ll smell a pungent aroma of sulfur or rotten egg.

Possible Diagnoses

A worn or failing catalytic converter is a likely cause of a P0420 code on your Accord. In fact, it’s the most common reason why you’d see this code on your reader with this particular make and model of car.

The catalytic converter’s job is to break down pollutants during the combustion process. It then turns them into gases that are less harmful to the environment. Ultimately, a problem with your catalytic converter of this nature means your vehicle is releasing an increase in harmful gases into the air.

Another possible reason for this code is a faulty oxygen sensor. This sensor in your Honda’s purpose is to measure the amount of unburned oxygen in your exhaust. It continuously communicates with the automobile’s electronic control system and will alter you when there’s too much oxygen exiting the vehicle’s exhaust system.

Rather than an error with the oxygen sensor (O2 sensor), it could also mean your oxygen ratio is either too high or too low, meaning your car is running too rich or too lean. Either way, it could contribute to a catalytic converter failure or other internal damage to your vehicle.

On the other hand, this code may mean you have an exhaust system leak or a misfire. A misfire could be the cause of catalytic converter failure.

You could take the time to experiment until you figure out the cause, especially if you’re a whiz with vehicles. However, if you only have basic auto repair skills, this could be a frustrating process.

Process to Fix the P0420 Code

This repair isn’t for beginners. It requires mechanic-level knowledge to fix. If not, it could be a highly frustrating process and result in an inaccurately fixed vehicle. Specifically, if you replace the catalytic converter and don’t address any misfire codes first, you could risk ruining the catalytic converter you just purchased and replaced. Besides the possibility of damage to a new catalytic converter, it’s also possible to invest a large amount of money when you use trial and error to fix the problem. This is because you could be repairing other parts needlessly before you find the source of the issue. It’ll also lead to a great deal of wasted time.

Use a Code Reader

First and foremost, before you delve into this repair, use a code reader to rescan for any additional errors. This is to ensure you fix any other problems before addressing this code.

Some codes to especially pay attention to at this point include any from P0300 through P0308. These all signify a misfire, and you must address that concern first before replacing a catalytic converter.

The following codes are also noteworthy codes to take care of before you replace a catalytic convert:

  • P0171
  • P0172
  • P0174
  • P0175

These signify the engine is running with either not enough or too much air. Running too rich can lead to engine damage, while running too lean can cause catalytic converter damage.

Check the Exhaust System

Next, check your exhaust system for any leaks or other damage. Look at the gaskets, exhaust pipes, and manifold. Repair any leaks or damaged components before proceeding. Once fixed, clear the code. You’ll need to wait for several drive cycles to determine for certain if the problem was fixed.

Replace the O2 Sensor

Make sure you run the reader to identify which sensor is failing. To fix this problem, you’ll need to lift the car on a jack. Next, identify where the faulty 02 sensor is. You’ll then need to disconnect the connection for your wiring harness.

Remove the sensor with an open-end wrench or 02 sensor socket. Then, loosen the oxygen sensor and remove it. Before replacing it, line the old one with your new 02 sensor and ensure it’s the correct one.

Once you verify it’s the correct one, install the new one and reattach the wiring harness.

Assess the Downstream 02 Sensor’s Function

When the vehicle is running and has reached its operating temperature, use a digital multimeter to evaluate the voltage from the downstream O2 sensor. With a functioning catalytic converter, the reading will be around 0.45V. On the other hand, a reading that’s switching between 0.9V and 0.1V means you need to replace the catalytic converter.

Replacing the Catalytic Converter

First get your vehicle onto the jack stand to replace the catalytic convert. Then, identify where your catalytic converter is. Next, remove the O2 sensor as well as its connections. You should never force this process, or you could cause damage. Penetrating oil will make this process much easier. Merely spray it on and allow it to sit for a few minutes. Then, try to remove it.

Finally, you can replace the catalytic converter, and it’s as easy as installing the new one and then replacing the 02 sensor. Be sure to replace the 02 sensor’s fittings as well.

After you complete this process, turn on your vehicle and wait for a few to see if your “Check Engine” light comes on. If it remains off, then you repaired the P0420 Honda Accord code problem successfully. If you still see the light, try running a scan for other codes besides P0420 because you may need to make another repair to fix the problem.

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