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P0420 Code: Catalyst Efficiency Below Threshold - Causes & Fix

By: CowboyBenWade

Interviewer: Could you explain the catalytics converters and the P0420 to us - why they fail, how they work, and why they are so expensive to repair?

CowboyBenWade: Of course, catalytic converters use catalysts (precious metals) to break apart nitrogen oxides (NOx) in the exhaust so they don't escape into the environment. Also, in the second stage, harmful carbon monoxide CO and hydrocarbons oxidize into CO2 and H2O.

What are nitrogen oxides and why are they bad?

Nitrogen oxides are nitrogen atoms bound to oxygen ions by the sharing of elections. This is called a covalent bond. These groups of atoms that are bound together through the sharing of electrons are called molecules. Molecules are the smallest compound that can partake in chemical reactions. Nitrogen oxides include nitric oxide (NO) and nitrogen dioxide (NO2), which are atmospheric pollutants. Nitrogen is an inert gas that is abundant in the atmosphere. It usually does not combine with oxygen, however, the combustion of fossil fuels in an engine causes nitrogen and oxygen to bond, forming NOx. The x stand of a number of oxygen ions that are bound to the nitrogen atom. Nitrogen oxides are highly reactive because they want to donate those excess oxygen ion and 'reduce' down to nitrogen and oxygen again. This is why they are called primary pollutants because they can break down to form 'secondary pollutants' like ozone.

How does engine exhaust produce ozone?

Ozone is three oxygen ions bound together - O3. Nitrogen oxides produce ozone when sunlight hits the NO2, splits the molecule, and releases a free floating oxygen atom. Oxygen that we breathe exists in the atmosphere as O2 - two oxygen atoms bound together. Free floating oxygen atoms don't exist long by themselves they want to bind with something to form oxides. Oxygen will bind with almost anything; it is is a highly reactive element because of its number of outer elections. When oxygen binds with iron, it forms rust. This process of oxygen binding to other things is called oxidation.

That free floating oxygen atom that sunlight split from the NO2, will bind with regular O2 oxygen molecules to form O3. Ozone is fine if it's high in the atmosphere, and there is a layer of ozone that protects the earth from ultraviolet rays. But ozone at ground level is bad, and that's why we want to reduce how much O3 we produce through the burning of fossil fuels.

What are catalysts and how do they work in catalytic converters?

Now that we understand what nitrogen oxides are, where they come from, and why they are harmful, we can understand how catalytic converters 'reduce' or beak them apart. Here is a short chemistry lesson on catalysts.

Catalysts are substances that speed up a chemical reaction without themselves being consumed or used up. In vehicle exhaust, you have these compounds like NOx that want to donate an oxygen atom. The atoms are bound together through the sharing of their outer electrons. However, they can break apart in a chemical reaction that causes one of the oxygen atoms to break away. This is called reduction reaction - a large molecule donates atoms and breaks down into smaller compounds. In this case NOx reduces to NO + O. Or this is what's happening in the environment without a catalytic converter. Catalytic converters force this reduction reaction to happen in the exhaust system using catalysts so that NOx reduces to compounds that we like such as H20 and O2 instead of reducing naturally in the environment to compounds we don't like such as O3.

A catalytic converter has two stages that facilitate two types of reduction reactions.

First Stage - Reduction Catalyst

The first stage of a catalytic converter is responsible for breaking apart nitrogen oxides NOx and converting them into oxygen and nitrogen. The first stage of the honeycomb structures is coated with platinum and rhodium - the catalyst. As the NOx passes over the platinum and rhodium, it reacts and donates its nitrogen atom to the platinum and rhodium elements. The oxygen that is left over moves into the second stage of the cat; we'll talk more about this later.

What happens to the nitrogen atom from the NOx that bound to the catalyst?

The nitrogen from NOx 'sticks' in the catalytic converter for a little while because it's bound to the catalyst at this point. The catalysts in the first stage accepts just one nitrogen atom from the NOx. NOx comprises of one nitrogen atom bound to two oxygen atoms. Nitrogen exists in the atmosphere as N2 (two nitrogen atoms bound together). So the nitrogen that get's trapped in the first stage of the cat waits around for a little while until it comes into contact with another nitrogen atom. At this point, the catalyst releases the nitrogen ion, it binds with the other free floating nitrogen atom to form N2, and passes out the exhaust. Then the cycle starts over again.

The catalyst just accepts and donates nitrogen atoms, therefore it's not getting used up in any way. As long as the platinum and rhodium cover enough surface area to allow the NOx to contact it, the catalytic converter will work efficiently. If a problem with the engine causes oil to leak into the exhaust, soot can build up on the honeycomb structure and cover up the platinum and rhodium catalyst. This would reduce catalyst surface area, and some of the NOx would have nowhere to donate its nitrogen atom, the P0420 would trigger, and it would pass into the environment.

Second Stage - Oxidation Catalyst

Remember all that oxygen that stripped away from the NOx in the first stage? It's useful in the second stage of a catalytic converter. This is called the oxidation phase because that oxygen that was stripped away in the first stage is used to convert carbon monoxide and hydrocarbons into carbon dioxide and H2O. Carbon monoxide is CO - one carbon atom and one oxygen atom. It can accept another oxygen atom to form CO2. We like CO2 more than CO; we breathe CO2 all the time, but breathing CO is fatal. Thanks to the reduction reaction in the first stage, we now have lots of free floating oxygen atoms in the seconds stage. The catalysts are platinum and palladium are used in the second stage to help with this oxidation reaction. The catalyst is hot and it just heats the oxygen and carbon monoxide so they react with each other. It's called an oxidation reaction because a compound CO is reacting with oxygen to form CO2.

Other oxidation reactions are happening in the second stage as well because there are other hydrocarbon byproducts besides carbon monoxide that need to be reduced into less harmful molecules. Some of these hydrocarbons oxidize into H2O (water) - notice the oxygen atom bound to two hydrogen atoms.

How efficient are catalytic converters?

When a vehicle is new, the catalytic converter is functioning around 99% efficiency, and after a few thousand miles it drops to 95%. This means that 95% of the harmful CO and NO are getting converted into CO2 and H2O, and the other 5% are escaping into the atmosphere. Sensors before and after the catalytic converter measure the harmful components, and the computer system expects them be reduced by this specific amount - the threshold. When it gets below that threshold, the P0420 (System Efficiency Below Threshold) code will appear, which usually happens around 92% efficiency.

Causes of P0420 and catalytic converter damage

As we discussed earlier, inside a catalytic converter is a honeycomb type structure that looks like screens stacked one after the other that run the length of the converter. It's coated in nanoparticles of precious metals (catalyst). This is called the 'substrate'. Keeping this substrate clean of carbon buildup soot, and gunk from the engine is imperative. If those nanoparticles accumulate carbon buildup, catalytic efficiency will plummet. Two things will happen - your exhaust gas will flow slower and cause a loss of power. This is because the tiny holes in the screen are getting even smaller due to carbon buildup, and it's restricting the flow of exhaust gas. Secondly, the P0420 code will show on your dash because the catalyst is covered up and unable to do its job.

3 main main causes of catalytic converter failure

Oil/soot in the exhaust - Under normal circumstances, only fuel and air should be combusting in the cylinder. Engine oil should not be burning in the cylinders, but it sometimes gets in through leaky piston rings, faulty gaskets, or through lubrication system in the turbocharger. The catalytic converter is designed to handle byproducts of air/fuel combustion - not the burning of oil. Engine oil produces a white smoke and thin soot when it burns. If this soot goes through the exhaust, it will collect on the honeycomb structure in the catalytic converter. Over time, it will accumulate and the catalytic converter will fail. The engine will experience a loss of power (because of poor exhaust flow), and it will fail emissions because the catalyst is clogged with carbon and cannot do it's job.
Overheating - This occurs when the catalytic converter gets too hot and the substrate (catalyst) melts. Because of bad timing, poor spark ignition, or leaking exhaust valves, unburned fuel can pass into the catalytic converter and combust, causing intense heat. The honeycomb material can melt, or it can just be covered in carbon from the burning fuel. This can clog the catalytic converter very quickly, back pressure will build in the exhaust, and soon the engine will have no power. In this case, a replacement catalytic converter will have the same issue. Have a mechanic diagnose the cause of the clogged converter, fix that, then replace the converter.
Physical damage - Hitting a curb or somehow causing physical damage to the housing can cause a P0420, but there's nothing wrong with your engine in this case. A visual inspection can quickly diagnose this issue. If the housing is torn open and exhaust gas is flowing out, it will make an audible noise called an 'exhaust leak'. If converter is severely dented, the inside may be cracked or broken. Exhaust gas can then pass through without encountering enough of the catalyst, the sensors will flag the lowered efficiency, and the code will show on the dash.

How to fix P0420

A related code, P0300 Misfire, can cause a P0420 and lead to a clogged or destroyed catalytic converter, so check for this code first. Next check the sensors.

Faulty oxygen sensors - These can trigger a P0420 when nothing is mechanically wrong. Aftermarket oxygen sensors fail often, so be sure to replace them with OEM sensors.
Leaking fuel injectors - These will often present with P0172 and P0300 codes. They can cause unburned gas to combust in the catalytic converter, heating it up, and eventually cooking the substrate.
Exhaust leaks - Exhaust pipes and manifolds have lots of welds, they get really hot when the car is running, and they get really cold when the car is off. The metal expands and contracts through these temperature extremes and cracks develop around welds. This is very common, and it can throw off the accuracy of oxygen sensors in the exhaust, resulting in P0420.

What is Bank 1 and Bank 2?

Some vehicles have two catalytic converters, and the 'Bank' specifies which one is throwing the code. Bank 1 is the side of the vehicle where cylinder number 1 is located. Bank 2 is the opposite of the engine. Check your owners manual or watch this video to find out which side of your car is bank 1 and bank 2.

Can I drive with a P0420?

You can drive with this code, but do something about it within a reasonable amount of time. The worst that can happen is that your exhaust will not be as environmentally friendly as some local municipalities would like it to be. If you live in an area that requires emissions checks, you will probably fail.

Why are catalytic converters so expensive to replace?

The precious metals platinum, palladium and rhodium are good at storing and donating oxygen atoms to the pollutants passing by. They are also durable enough to last the lifetime of the car. However, only a small amount of these metals are needed because they are sprayed as nanoparticles on the honeycomb structure (substrate) inside the catalytic converter. Coating the honeycomb structure in precious metal nanoparticles increases the surface area of catalyst so more of pollutants can react. As stated earlier, even with this large surface area, the reaction is only around 95% effective, but that's pretty good.

Catalytic converters are still expensive even with the small amount of precious metals they contain. Most catalytic converters get recycled. However, they do 'leak' precious metals onto the highway, and it's possible to get bead of platinum from a cup of dirt taken from the side of the road as detailed in this experiment from Popular Mechanics.

About the Author

CowboyBenWade does interviews and lectures on a huge variety of topics, and he is an expert in a multitude of fields. His knowledge includes but is not limited to: engineering, chemistry, aviation, cooking, and farming.

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