What happens to exhaust emissions if the catalytic converter is gutted?

• A. HC and CO would be elevated; CO2 would be higher than normal; O2 would be lower.
• B. HC would decrease while CO would increase.
• C. All emissions would remain unchanged.
• D. The HC and CO would be elevated, CO2 would be lower than normal, and O2 would be slightly higher.

Answer: (D)

The catalytic converter’s job is to clean exhaust by oxidizing HC and CO into CO2 and H2O, and to help reduce NOx. When it’s gutted, those aftertreatment reactions don’t occur, so unburned hydrocarbons and carbon monoxide pass through in higher amounts. Because the converter normally helps convert CO to CO2, removing it means less CO2 is produced in the exhaust stream, so CO2 levels drop. The catalytic reactions also consume some oxygen, so with no converter, there’s slightly more O2 left in the exhaust.