If an oxidizing catalytic converter on a diesel engine failed, which emissions would likely increase?

• A. Nitrogen
• B. HCs and carbon monoxide
• C. Oxides of nitrogen
• D. HCs and carbon dioxide

Answer: (B)

An oxidizing catalytic converter works by converting unburned hydrocarbons and carbon monoxide in the exhaust into carbon dioxide and water, using catalytic sites that promote oxidation at the exhaust temperatures. In a diesel engine, there are notable amounts of HC and CO left from incomplete combustion, so this device significantly reduces those emissions when functioning properly.

If the oxidizing converter fails, that oxidation step is lost, so hydrocarbons and carbon monoxide pass through largely unreacted. Their levels in the exhaust would rise because there’s no catalyst present to convert them to CO2 and H2O. Oxides of nitrogen, on the other hand, are not targeted by an oxidizing converter; they’re typically addressed by separate systems like SCR or other NOx-control technologies, so merely losing the oxidation function does not inherently cause NOx to increase.