As car owners, car problems strike fear into our hearts. Cars are not only our means of transportation, but most of us see them as family members. So when a check engine light code like the P0420 pops up, it’s quite alarming. .

The diagnosis process comes with many roadblocks. Trouble codes help us laser in on the system that is causing the issue and sometimes even point directly to the failed component. Without them, we would have to rely on rudimentary methods to diagnose our vehicles of ever-increasing complexity.

This is why we’ve put together this guide, specifically designed to help diagnose and fix the P0420 code. We’ll also explain all the symptoms, causes, and anything else you may need to know about this trouble code.

What is the P0420 Code, and What Does It Mean? 

The P0420 code (Catalyst System Efficiency Below Threshold (Bank 1)) means that the ECU has detected that the catalytic converter downstream of the first bank of cylinders is underperforming.

In cars with two cylinder banks (V6, V8), the P0420 code refers to the bank with the first cylinder. An inline-six engine will use this code for the first three cylinders. Check your car’s service manual to see which cylinders belong to which bank if you’re unsure. 

What is a Catalytic Converter?

A catalytic converter is a device attached to the exhaust. It converts noxious gasses from the engine into carbon dioxide, steam, and less harmful gasses. Back in the mid ‘70s, the world came together to assess the environmental damage we were causing by burning fossil fuels. It was decided that exhaust gasses resulting from combustion were simply too toxic to be released into the atmosphere. 

To combat this issue, various environmental governing bodies came up with a device that would essentially catch the majority of toxic gasses and prevent them from ever leaving the exhaust system. The catalytic converter is by far the most important part of that system. Catalytic converters feature a honeycomb structure made of precious metals that stands in the way of the exhaust gasses as they pass through the system. 

By the time they reach the end of the catalytic converter, many of the toxic molecules are split, turning them into fairly safe compounds. Of course, no catalytic converter will be able to eliminate 100% of the toxic gasses, but most are fairly efficient at their job. 

Another important piece of this puzzle is the O2 sensor. 

What is an Oxygen Sensor and How Does it Work? 

An oxygen (O2) sensor measures the amount of oxygen coming through the exhaust system. The PCM uses this information to adjust the air-fuel mixture. There are usually two O2 sensors per exhaust manifold line. One is placed right after the exhaust manifold and before the catalytic converter, while the other is positioned downstream of the catalytic converter. 

The former measures the amount of oxygen that exits the combustion chambers, while the other measures how much oxygen is found in the gasses once they pass the catalytic converter. The reason why we’re mentioning O2 sensors is because the downstream one can easily trigger the P0420 code. 

What Are the Common Symptoms of a P0420 Fault Code?

Although the symptoms of a P0420 code vary and can be caused by a fair amount of other issues, there are few that will direct you toward the root cause of the code. These include the following:

• Check engine light
• Reduced fuel economy
• Misfires
• A lean or rich air-fuel mixture
• Foul exhaust smell

Check Engine Light

This is where all your problems start, so it’s where we will start as well. A P0420 code will cause the check engine light to illuminate, but will usually come with a few other engine codes, as the cause is often not the catalytic converter.

Reduced Fuel economy

A clogged catalytic converter will reduce airflow and cause a host of problems, including reduced fuel economy. The exhaust gasses recirculating in the engine can cause the vehicle to run rich and thus consume fuel.

Foul Exhaust Smell

A telltale sign that you have a P0420 issue is a rotten egg smell coming from the exhaust. Although most sulfur (the chemical responsible for the smell) is filtered out during the fuel refinement process, it can still be present in unleaded gasoline, especially in diesel.

What Are the Common Causes of a P0420 Fault Code?

Unfortunately, most engine and transmission issues will filter down to the catalytic converter. These issues almost always cause increased emissions and can make diagnosis difficult. Some of the common ones are:

• Wiring damage
• Exhaust system damage (manifold and pipe)
• Incorrect fuel
• Faulty O2 sensor 
• Failed or missing catalytic converter
• Misfiring cylinders

Wiring Damage

Oxygen sensor wires can cause a false reading to be sent to the ECU. This can in turn cause several codes, including the P0420. Wiring damage is common in areas with a large presence of rodents. As it turns out, rodents simply love to munch on wires and the oxygen sensor is especially vulnerable because of its location.

Exhaust System Damage

Damage to the exhaust is never good. It’s also fairly common in older vehicles and colder climates that use salt on roads. Cracks in the exhaust manifold or holes in the exhaust can cause the P0420 code.

Incorrect Fuel

Incorrect fuel can cause several major issues, including the P0420 code. Other than causing damage to the inner components of the catalytic converter, putting the wrong fuel in your car can clog up the cat. 

Faulty O2 Sensors

As the O2 sensors measure the oxygen content before and after entering the catalytic converter, if either of them fails, you will get a P0420 code. Incorrect information from the O2 sensor can cause the air-fuel ratio to run rich or lean

Failed or Missing Catalytic Converter

One of the most common causes of this trouble code is the failure of the catalytic converter. Sometimes, it may even be missing completely, as they are highly prized by thieves. In the event that you experience an engine issue that causes oil to mix with fuel, it’s likely to damage the inner substrate of the catalytic converter.

Misfiring Cylinders

A failed catalytic converter can cause cylinders to misfire and it can happen the other way round too. A cylinder misfire where one or more cylinders is not burning the fuel often causes that unburnt fuel to be dumped into the catalytic converter which overtime can cause physical damage to the sensitive internals.

How to Diagnose the Cause of P0420 Code 

Getting to the root cause of a P0420 code is no easy feat. However, because the common causes are easy to test for, you might find yourself back up and running as soon as the parts roll in.

Inspection Test

The most common causes of the P0420 code lie under the car. Rocks and corrosion can damage the exhaust system and catalytic converter. Jack up your vehicle safely before inspecting for damage.

An important tool in diagnosing whether your car has a more serious issue is whether the car is running rich or lean. This can mean that the oxygen sensor has failed, but it can also mean that there is a deeper issue with another intake-related sensor or even the fuel injectors.

Your car will produce consistent black smoke if the air-fuel mixture is rich. Running lean usually means bad starting behavior or stalling altogether. Excess fuel can cause catalytic converter failure, so it is best to solve the underlying issue before replacing your cat.

Of course, you can listen for catalytic converter issues. Typically, a failed catalytic converter will make rattling noises as you are driving and when you start the vehicle. This is because as exhaust gasses try to pass through it, the broken pieces will shake around. You can also remove the catalytic converter and check the inner parts for any carbon deposits. 

Wiring Damage

The most concrete way to test the wiring system is to not only inspect the harness for damage, but use a multimeter to test it. The signal wire outputs around 5V and the live 12V. These numbers do differ slightly when the car is on and running so make sure to take that into account when testing.

That being said, if the voltage is extremely low or high, it likely means there is an issue. Checking the battery and PCM will also help, but those are unlikely failure points.

ODB II Reader

An ODB II reader is essential in diagnosing this issue. For starters, it will tell you that you have a P0420 code. From there, you’ll be able to move to diagnose the system directly.

You can also read the live data from oxygen sensors. The voltage should read around 0.1V when the car is running lean and around 1V when running rich. The wavelength should fluctuate drastically when the car is running and stay consistent when the car is off.

Downstream O2 Sensor

The oxygen sensor feeds oxygen data to the ECU, which controls the air-fuel mixture. There is an O2 sensor on the exhaust manifold that reads how much oxygen the engine is outputting through the exhaust. 

It then compares this to the sensor that sits right after the catalytic converter. If the readings are similar, it will register a fault. 

An oscilloscope is your friend when testing the multiple O2 sensors your car may have. Now, you need to back probe the sensor while it’s still connected. A metal paper clip can help, but if you can get in there with your positive (red) probe, that can work too. 

Your ground (black) probe needs to be connected to a metal part of the vehicle. Once you’ve completed that, you need to turn on and warm up your car. We warm up the car to let the oil circulate and to let the engine reach operating temperature.

Now that you’re sufficiently convinced that the car is warmed up, keep the car at around 2,000 RPM. The oscilloscope should read a square wavelength of between 0.1V and 1V.

Underlying Engine Issues

As mentioned, many engine issues can cause catalytic converter failure and the P0420 code. Each engine issue deserves its own guide, but if after you’ve replaced your catalytic converter or O2 sensor, there are still engine codes, you might be dealing with a much larger underlying issue. As a result, even if you change all these components we’ve already mentioned, you might end up with the same problem down the road.

How to Fix a P0420 Code

There are many ways to fix the root cause of this trouble code. Of course, if the oxygen sensor or exhaust system has failed, those would need replacing. However, with a clogged catalytic converter, using a cleaner may resolve the issue.

You should not expect miracles. A catalytic converter cleaner will not fix broken inner parts, and it’s likely that you will have to replace the cat even after using the cleaner. 

Many outlets recommend using soap and water to clean your catalytic converter. This will most likely damage the inner parts further or at least ensure that they will corrode and fail down the line.

Does P0420 Clear Itself?

Once you’ve rectified the root cause of the issue, you can go about clearing the code. If you don’t have access to an ODB II reader to clear the codes, it should clear itself within a couple of drives.

Is It OK to Drive with a P0420 Code?

A P0420 code is a serious one. Although it will not cause an immediate safety issue, it can  indirectly damage your engine if you ignore it for too long. As soon as it is safe to stop, you should not drive your vehicle any further.

Solve the P0420 Code Using Quality Parts 

If you own a European car and you’ve found yourself with a P0420 code illuminating your dashboard, you’re in the right place. Whether it’s an O2 sensor that’s causing your issues, or some kind of misfiring problem, we have the parts you’ll need to fix your car. 

Here at eEuroparts.com, we offer one of the largest catalogs of Genuine, OEM and aftermarket parts for a variety of European vehicles. Check out our store section to find products that match your vehicle!