Do diesel engines have catalytic converters?

With the tremendous increase in pollution levels, all governments have taken stringent guidelines towards environmental pollution, most of it coming from vehicles. These pollutions are primarily; carbon monoxide, nitrogen oxides, and hydrocarbons.

Today, car manufacturers are supposed to meet the National Emissions Standards Act on non-toxic emissions. A catalytic converter is an emission control device found on the exhaust system of your vehicle.

The device reduces the emission of toxic fumes and gases from your car’s engine by converting them into less toxic, environmentally-friendly gases. Keep leading to find out more about catalytic converters and if they are found in diesel cars.

So, Do Diesel Cars Have Catalytic Converters?

Yes! Diesel cars have a catalytic converter but not the same as that petrol or gasoline motor vehicles. Today, most Diesel engines have emission controls that consist of four stages; Exhaust Gas Recirculation (EGR), Diesel Particulate Filtration (DPF), Diesel Oxidization Catalyst (DOC), and Selective Catalytic Reduction (SCR). As suggested by the names, the two stages referred to as catalytic converters include the DOC and the SCR. This is because of their use of catalysts in facilitating chemical reductions to decrease vehicle emissions.

Most Diesel cars and engines can use the two-way catalytic converter. The two-way oxidation cat only consists of the DOC that functions to convert carbon monoxide to CO2 and hydrocarbons that are changed to water and carbon dioxide.

More innovative three-way catalytic converters are installed into modern cars. They have a two-way converter DOC to oxidize carbon monoxide. Still, They are also fitted with an SCR reduction catalyst to reduce nitric oxide, and nitrogen dioxide emissions together referred to as NOx.

How Does A Diesel Catalytic Converter Work?

The catalytic converter carries out the essential function of converting harmful compounds into non-toxic products. The interior of its metal casing contains two ceramic blocks with thousands of micro-cellular units resembling a honeycomb.

The ceramic blocks are coated with metals such as palladium and platinum. Due to its close location to the engine, the catalytic converter usually heats up and catalyzes chemical reactions that convert toxic gases to harmless byproducts before their emission into the atmosphere.

The Diesel Oxidation Catalyst (DOC) functions almost similarly to the petrol engines. They catalyze the reduction of carbon monoxide to water and carbon using metal catalysts. They also break down volatile byproducts and hydrocarbons into less harmful chemicals preventing the emission of harmful gases.

However, different from petrol-based catalytic converters, the diesel-based cat has no rhodium, more so, the diesel cat comes with an attached diesel particulate filter (DPF) to eliminate the particulate matter absent in petrol-based petrol catalytic converter.

Also known as DEF, the SCR catalyzes the breakdown of Oxides of Nitrogen (NOx) vehicle emissions using an ammonia-based catalyst and converts them into Oxygen and Nitrogen. As the reductant and exhaust go over the SCR catalyst, a chemical reaction occurs, converting NOx emissions to water and nitrogen.

The open-loop SCR system is about 75 to 90 percent effective in reducing NOx emissions. In comparison, the closed-loop system found on stationary engines can achieve an effectiveness of more than 95 percent. SCR also reduces hydrocarbon emissions by about eight percent. The use of low-sulfur fuel can improve its performance.

Video Overview

Diesel catalytic converter vs Diesel Particulate Filter (DPF)

Diesel engine converters are also uniquely designed to work with diesel engines to target and filter particulates. Diesel engine emissions consist of particulate solids and matter and the toxic, unburned fuel emitted from the engine through the cat in the form of vapor and gas. These particulate matters are visible in the form of soot and occur in black smoke often seen from diesel engines. This usually causes the brown cloud of smog that surrounds industrialized cities.

Particulate matter consists of three primary components. These include; soluble organic fraction (SOF), lubricating oil hydrocarbon (LHC), and fuel-derived hydrocarbon (FHC). The lower Sulphur diesel fuel has gradually lowered the amount of particulate matter in the atmosphere. However, there is no Sulphur free diesel, and there is still considerable amounts of particulate matter due to hydrocarbons.

But luckily, particulate matter is solid, meaning it can be filtered. Similar to the functioning of a diesel catalytic converter, a Diesel Particulate Filter (DPF) does the job of removing the solids from the diesel combustion emissions before it escapes into the atmosphere. It consists of a matrix of a composite including metal fibers, silicon carbide, or cordierite.

How Long Does A Diesel Catalytic Converter Last?

A diesel cat commonly lasts for about ten years or 100 000 miles. However, this will also depend on the length of the trips that the vehicle makes. A vehicle that makes short start-stop journeys could see the catalytic converter wear out quicker than one that makes long, consistent trips.

This is because the engine is switched off before the catalytic converter attains the optimum temperature range to catalyze the reactions fully. Although they usually have a relatively long lifespan, a catalytic converter still requires to be checked occasionally for external and internal damage. If it gets damaged, it’s costly to replace.

What Are The Signs Of A Bad Catalytic Converter?

A catalytic converter can become defective over time due to various reasons such as the buildup of carbon deposits, restricting the proper flow of gases out of the exhaust. Other possible reasons can include overheating, physical damage, and contamination.

Some of the signs of a defective catalytic converter include;

  • decreased acceleration,
  • slow engine performance,
  • dark smoke from the exhaust,
  • the smell of rotten eggs or Sulphur from the exhaust,
  • excessive heating under the vehicle.

Modern cars have oxygen sensors that light red if the converter is not functioning correctly. If you notice any of these symptoms, you should go for a mechanical inspection immediately.


Catalytic converters play a significant role in vehicles by preventing the emission of harmful gases into the atmosphere.

Today, most cars are also fitted with diesel gas particulate, exhaust gas recirculation, and Ad Blue selective catalytic reduction technology that supplement the catalytic converter to clean the car exhaust emissions further. However, you must have the cat checked regularly to maintain maximum performance.

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