Yes, a car can run without a catalytic converter. The engine will start and the vehicle can move. However, doing so is illegal for street use in the United States, will cause your vehicle to fail emissions testing, and leads to increased harmful emissions. You'll also likely experience a lit check engine light and notice a strong smell of sulfur or rotten eggs from the exhaust due to unprocessed hydrogen sulfide.
The primary role of the catalytic converter is to act as an exhaust filter. It uses precious metals like platinum, palladium, and rhodium to catalyze a chemical reaction that converts harmful pollutants—carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx)—into less harmful carbon dioxide (CO2) and water vapor. Without it, these toxins are released directly into the air.
Beyond the legal and environmental issues, driving without a "cat" can have downstream effects. The engine control unit (ECU) relies on data from oxygen sensors located before and after the converter. A missing converter will cause these sensors to send abnormal readings, confusing the ECU. This can result in poor fuel economy, rough idling, and reduced engine performance over time. While the vehicle is operational, it is not operating as intended by the manufacturer. The only legal and practical reason to remove a catalytic converter is for off-road or competitive racing purposes, where the vehicle is not driven on public roads.
