Negative aspects

Some early converter designs greatly restricted the flow of exhaust, which negatively affected vehicle performance, driveability, and fuel economy. Because they were used with carburetors incapable of precise fuel-air mixture control, they could overheat and set fire to flammable materials under the car.

Removing a modern catalytic converter in new condition will not increase vehicle performance without retuning, but their removal or “gutting” continues. In such cases, the converter may be replaced by a welded-in section of ordinary pipe or a flanged “test pipe” ostensibly meant to check if the converter is clogged by comparing how the engine runs with versus without the converter, which facilitates reinstallation of the converter in order to pass an emission test.

In many jurisdictions, it is illegal to remove or disable a catalytic converter for any reason other than its direct and immediate replacement. In the United States, for example, it is a violation of Section 203(a)(3)(A) of the 1990 Clean Air Act for a vehicle repair shop to remove a converter from a vehicle, or cause a converter to be removed from a vehicle, except in order to replace it with another converter, and Section 203(a)(3)(B) makes it illegal for any person to sell or to install any part that would bypass, defeat, or render inoperative any emission control system, device, or design element.

Vehicles without functioning catalytic converters generally fail emission inspections. The automotive aftermarket supplies high-flow converters for vehicles with upgraded engines, or whose owners prefer an exhaust system with larger-than-stock capacity.

Warm-up period

Vehicles emit most of their pollution during the first five minutes of engine operation before the catalytic converter has warmed up sufficiently to be effective.

In 1999, BMW introduced an electrically heated catalyst, which they called “E-CAT”, in their 750iL sedan. Heating coils inside the catalytic converter assemblies are electrified just after engine start, bringing the catalyst up to operating temperature very quickly to qualify the vehicle for low emission vehicle (LEV) designation.

Environmental impact

Catalytic converters have proven to be reliable and effective in reducing noxious tailpipe emissions. However, they also have some shortcomings and adverse environmental impacts in production:

  • Although catalytic converters are effective at removing hydrocarbons and other harmful emissions, they do not reduce the emission of carbon dioxide (CO2) produced when fossil fuels are burnt.
  • Carbon dioxide produced from fossil fuels is one of the greenhouse gases indicated by the Intergovernmental Panel on Climate Change (IPCC) to be a “most likely” cause of global warming.
  • The U.S. Environmental Protection Agency (EPA) has stated automobile emissions are a significant and growing cause of global warming, because of their release of nitrous oxide (N2O), a greenhouse gas over three hundred times more potent than carbon dioxide. The EPA states that motor vehicles contribute approximately 8.2% of anthropogenic nitrous oxide emissions in 2008, from a high of 17.77% in 1998.
  • Nitrous oxide makes up 7.2% of greenhouse gases.
  • An engine equipped with a three-way catalyst must run at the stoichiometric point, which means more fuel is consumed than in a lean-burn engine. This, in turn, means relatively more CO2 emissions from the vehicle. Nevertheless, catalyst-equipped engines produce cleaner exhaust than lean-burn engines.
  • Catalytic converter production requires palladium or platinum; part of the world supply of these precious metals is produced near Norilsk, Russia, where the industry (among others) has caused Norilsk to be added to Time magazine’s list of most-polluted places.