What is a Catalytic Converter and what does it do?

Learn what a catalytic converter does and how it works

The catalytic converter is a critical component in the automotive exhaust system, designed to reduce harmful emissions produced by an internal combustion engine. Introduced in the mid-1970s, catalytic converters have been pivotal in decreasing air pollution and meeting stringent environmental regulations worldwide. This article delves into the finer details of what a catalytic convert does and how it works to change harmful emissions into less harmful ones.

What does a catalytic converter do?

A catalytic converter’s primary purpose is to convert harmful pollutants in exhaust gases into less harmful emissions before they leave the vehicle’s exhaust system. The key pollutants targeted by catalytic converters include carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx). These pollutants are byproducts of the combustion process and pose significant health and environmental risks. Carbon monoxide is a toxic gas, hydrocarbons contribute to smog formation, and nitrogen oxides can lead to respiratory problems and acid rain.

A typical catalytic converter consists of three main parts: the core, the wash coat, and the catalyst

• Core— The core, also known as the substrate, is typically made of ceramic or metal and features a honeycomb structure. This design maximizes the surface area for the exhaust gases to interact with the catalyst. The honeycomb structure allows for a large contact area within a compact space, ensuring efficient conversion of pollutants.

• Washcoat—  The washcoat is a rough, porous layer that coats the core, increasing the surface area even further. It serves as a base for the catalyst and helps disperse it uniformly across the substrate. The washcoat is made from materials like alumina, silica, or other metal oxides.

• Catalyst— The catalyst itself is composed of precious metals such as platinum, palladium, and rhodium. These metals are chosen for their ability to withstand high temperatures and their catalytic properties. The catalyst facilitates the chemical reactions that convert harmful pollutants into less harmful emissions.

This is a typical catalytic converter showing the ceramic structure

How a catalytic converter works

Catalytic converters operate through a series of chemical reactions within the catalyst. The process involves three primary reactions:

• Oxidation of Carbon Monoxide (CO) to Carbon Dioxide (CO₂)— 2CO+O2→2CO2. This reaction involves the oxidation of carbon monoxide, a toxic gas, into carbon dioxide, a less harmful gas.

• Oxidation of Hydrocarbons (HC) to Carbon Dioxide (CO₂) and Water (H₂O)— CxHy+O2→CO2+H2O. Hydrocarbons, which are unburned or partially burned fuel, are oxidized into carbon dioxide and water. This reaction helps in reducing smog-forming emissions.

• Reduction of Nitrogen Oxides (NOx) to Nitrogen (N₂) and Oxygen (O₂)— 2NOx→N2+O2.
Nitrogen oxides are reduced to nitrogen and oxygen, both of which are harmless. This reaction is critical for reducing the formation of ground-level ozone and acid rain.

These reactions occur simultaneously as the exhaust gases pass through the catalytic converter. The converter’s effectiveness depends on maintaining optimal operating temperatures, typically between 400°C and 800°C (752°F and 1472°F).

Two types of  Catalytic Converters

Two main types of catalytic converters are two-way (oxidation) and three-way (oxidation-reduction) converters.

1. Two-Way Catalytic Converters: These are primarily used in diesel engines and older gasoline engines. They facilitate the oxidation of carbon monoxide and hydrocarbons into carbon dioxide and water. However, they do not reduce nitrogen oxides.
2. Three-Way Catalytic Converters: These are the standard in modern gasoline engines and perform three simultaneous tasks: oxidizing carbon monoxide, oxidizing hydrocarbons, and reducing nitrogen oxides. The three-way catalytic converter is highly efficient in reducing all three major pollutants.

How long a catalytic converter lasts

A catalytic converter causes a catalytic reaction. There’s nothing mechanical inside. There are no wear items inside. So, the answer is that it depends on how you treat it. There are thousands of vehicles on the road with over 300K miles, and they are running on the factory converter.

Here’s the bottom line: Catalytic converters do NOT wear out; they are killed by owner negligence. That’s right: YOU kill your catalytic converter by ignoring misfires, not fixing oil and coolant leaks, and hitting things with the undercarriage.

What causes a Catalytic Converter to Fail?

I’ve written another article that describes what causes a catalytic converter to fail. You can find it here

An engine misfire is the number one cause of catalytic converter failure. When a cylinder misfires, it dumps raw fuel into the converter causing it to overheat as the converter tries to oxidize the excess fuel. If the misfire continues long enough, the converter can overheat to beyond 1,300 degrees, causing the ceramic honeycomb to melt down. That plugs up the openings in the honeycomb restricting exhaust flow and reducing the engine’s ability to breathe. A meltdown can also cause the honeycomb “brick” to shatter, causing a catalytic converter rattle sound. A converter can also be damaged by impact or contamination from a leaking cylinder head gasket that sends coolant into the exhaust.

How much does a catalytic converter cost

The cost of a new catalytic converter depends on where you buy the unit. Car makers are required to place a long warranty on their catalytic converters. In fact, the law states that an OEM catalytic converter warranty must be 80K miles or eight years. So OEM units are larger and contain more precious metals than aftermarket units. Aftermarket converters aren’t held to such a stringent standard; they are warranted for only 25,000 miles.

Aftermarket catalytic converters come in two styles, direct and universal fit. A direct fit catalytic converter is made to fit exactly onto your car’s exhaust system with no modifications. If your old converter had a flange to bolt directly to the exhaust manifold, a direct fit will also have that flange. A universal converter will not have any of those fitting. You will have to weld or bolt on all the adapters to make it fit.

Walker is one of the better manufacturers of aftermarket converters you can buy for your vehicle. Let’s use an example of a 2005 Dodge Grand Caravan with a 3.8L engine. A factory converter from the dealer lists for $642 plus a $200 core charge (they want the old converter because it can be recycled). However, an aftermarket converter costs $180 to $250. When you consider the difference in the warranties, you’re pretty much getting what you pay for. Since the OEM catalytic converter must be covered for 80K miles and the aftermarket converters only 25K miles, it makes sense that the factory units cost 3X more. The question for you, unless a catalytic converter is abused, it can last the life of the vehicle. They’re either destroyed by misfires, oil and coolant leaks, or impact. If you’re one of those people who drive around with your check engine light on, you may be having misfires without knowing. So you’re self destructing the catalytic converter. In that case, why would you spend the money for an OEM version? You’ll just destroy that too.

Can you clean a catalytic converter?

There’s a guy on YouTube with videos suggesting that you can clean your catalytic converter using two methods. His first method is to dump chemicals in your tank with the theory that they will clean off deposits on the honeycomb. If that doesn’t work, he suggests removing the catalytic converter and soaking it in dishwashing soap and water overnight. No car maker recommends either approach. First, dumping Xylene or MEK into your fuel tank can sometimes destroy the plastic and rubber in the fuel pump, fuel level sensor, fuel pressure regulator, and the fuel injector windings insulation. Keep in mind that raw fuel flows around the winding in the fuel injectors—that’s what cools them. If you dissolve the lacquer insulation on those windings, you destroy the injector. So there is REAL risk in follow his advice. Maybe he’s done it and gotten away with it. But if the plastics, rubber, and fuel injector windings in YOUR vehicle aren’t compatible, you’ll destroy them, making the cost of a new catalytic converter look like peanuts compared to a new fuel pump module and new injectors.

Can you remove your catalytic converter?

Ok, you’re a cheapskate, and you don’t give a damn about the environment. So you want to replace the catalytic converter with a straight pipe, right? Or, you want to chip out the ceramic honeycomb and run the catalytic converter empty. First off, what you propose is illegal. No legitimate shop will do it because they face a $10,000 find. Second, if you do it yourself, and your vehicle is equipped with a post cat oxygen sensor, you will get a check engine light—it’ll be on full time so you’ll never know if the computer has set a different code.

©, 2015 Rick Muscoplat

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