Yes, a car can operate without a catalytic converter, but it is illegal for street use in the United States and most other countries, will cause it to fail an emissions test, and is harmful to the environment. The vehicle will run, but you'll likely notice a louder exhaust note, a strong smell of gasoline from unburned hydrocarbons, and potentially a check engine light due to the missing downstream oxygen sensor data.
The catalytic converter is an emissions control device installed in the exhaust system. Its job is to convert harmful pollutants from the engine's exhaust—namely carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx)—into less harmful gases like carbon dioxide (CO2), nitrogen, and water vapor through chemical reactions. While the engine's core mechanical functions (combustion, power generation) are independent of the converter, its absence has significant consequences.
Driving without one is primarily a legal issue. The Clean Air Act makes removing a properly functioning catalytic converter a federal offense. The environmental impact is severe, as the car will emit vastly higher levels of smog-forming and toxic pollutants. The following table illustrates the dramatic difference in emissions with and without a functioning catalytic converter.
| Pollutant | With Catalytic Converter | Without Catalytic Converter | Increase |
|---|
| Hydrocarbons (HC) | 0.25 grams/mile | 8-10 grams/mile | ~3,200% |
| Carbon Monoxide (CO) | 3.4 grams/mile | 80-100 grams/mile | ~2,500% |
| Nitrogen Oxides (NOx) | 0.4 grams/mile | 4-5 grams/mile | ~1,000% |
Mechanically, the engine will run, but modern vehicles rely on data from oxygen sensors before and after the converter to fine-tune the air-fuel mixture. Removing it can confuse the engine's computer, potentially leading to rough idling or reduced fuel efficiency. The only legal and practical use for a car without a catalytic converter is as a dedicated off-road or track-only vehicle.
