Which after-treatment system is used to reduce NOx emissions and must be visually inspected?

• A. Diesel particulate filter
• B. Catalytic converter
• C. Diesel Oxidation Catalyst (DOC or OC)
• D. NOx adsorber

Answer: (D)

The main idea here is that some diesel after-treatment devices are dedicated to reducing NOx emissions, and those devices are specifically identified for visual inspection in many smog-check programs. The NOx adsorber, also known as a lean NOx trap, is designed to lower NOx in the exhaust by storing NOx during lean operation and then periodically releasing and reducing it during a regeneration cycle. This makes it the component whose primary purpose is NOx reduction, which is why it’s the one that must be visually inspected.

In contrast, a diesel particulate filter targets soot and particulates, a Diesel Oxidation Catalyst oxidizes CO and hydrocarbons, and a general catalytic converter reduces pollutants but isn’t specifically dedicated to NOx reduction. Since the question focuses on reducing NOx and the visual-inspection requirement, the NOx adsorber best fits both criteria.