Cars and trucks that burn fuels also produce emissions that are toxic to humans and the environment. For this reason, modern exhaust systems have been equipped with catalysts to convert pollutants from the engine into less harmful emissions.

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A catalytic converter is essentially a tube with a honeycomb structure inside that is coated with metals such as palladium or platinum. When hot gases pass through it, it creates a chemical reaction that turns carbon monoxide into carbon dioxide. It also helps break down fuel that hasn’t been burned in the engine.

Automakers sell both gasoline and diesel cars with catalytic converters. However, gasoline and diesel engines work differently and produce different emissions. A diesel engine uses glow plugs rather than spark plugs and requires a higher compression ratio and leaner air-fuel ratio to ignite. It also has a higher combustion temperature and produces toxic nitrogen oxides and more particulate matter than a gasoline engine. Therefore, the same catalytic converter and exhaust system will not work for both types of engines.

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Because diesel engines emit more soot and nitrogen oxides than gasoline engines, their exhaust systems are more complex. A modern system includes a diesel oxidation catalyst that converts carbon monoxide into carbon dioxide and breaks down hydrocarbons that weren’t burned in the engine, like a gasoline-powered vehicle does. But wait—there’s more.

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In order to meet increasingly stringent emissions standards, light diesel trucks such as pickup trucks were required in 2008 to use additional filtering technologies to capture soot and ash particles before they were released into the air. The diesel particulate filter (DPF) was invented by the Peugeot company to filter particles from the exhaust and reduce these emissions by 80%.

The final stage of the emissions control process is handled by the selective catalytic reduction (SCR) unit, which uses a urea-based diesel exhaust fluid (DEF) to convert NOx into nitrogen, carbon dioxide, and water vapor. Diesel trucks without SCR have an exhaust gas recirculation valve that helps reduce NOx by redirecting the exhaust gases into the engine’s air intake to burn off any leftover combustibles in the engine. With all this going on underneath a pickup truck, removing the catalytic converter is not a simple process.

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Drivers may be tempted to remove a catalytic converter system because it is so complex: it has more parts that can fail if not properly maintained or due to manufacturing or installation issues. A common problem that can affect diesel engine performance is when the DPF becomes clogged, restricting the flow of exhaust gases. Some drivers believe these systems reduce power and torque and affect fuel economy, so they remove them or install defeat devices to bypass them. This may have been helpful on older diesel engines that did not use SCR, which can actually reduce fuel economy.

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Some drivers remove emissions controls so they can “roll coal” – meaning they release large amounts of exhaust smoke into the air. According to the Environmental Protection Agency, removing or bypassing even a few controls can increase the amount of pollutants released. Completely removing the exhaust system from a heavy-duty diesel pickup truck can cause that one truck to emit as much particulate matter, carbon monoxide, NOx and unburned fuel as 300 trucks that still have such a system. It can also make the engine louder because the catalytic converter acts like a sort of muffler.

First, it’s illegal. As the EPA reminded everyone in a memorandum in 2020, Section 203(a)(3)(A) of the US Clean Air Act (CAA) “prohibits tampering with emissions controls, including those controls located in the engine (e.g., fuel injection, exhaust gas recirculation) and those controls located in the exhaust (e.g., filters and catalysts).”

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It can also void the truck’s warranty. For these reasons, the manufacturer might refuse to cover other, expensive problems that need to be repaired or replaced and are normally covered under warranty. Removing the catalytic converter or other components will obviously cause a truck to produce more pollution. That’s bad enough, but it can also cause the truck to fail vehicle inspection in a state like California and become unlawful for road use.

Finally, a partial or poor removal could even affect the performance of the engine, as it is factory-set to use these controls and read data from system sensors to run efficiently – which would defeat the purpose. Instead, look for more environmentally friendly and safer options to increase your vehicle’s performance, such as a cat-back exhaust or cold air intake.

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