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

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

Answer: (B)

The main idea is that NOx emissions are controlled by a device designed specifically to trap and then reduce NOx during regeneration. The NOx adsorber serves this purpose by capturing NOx when the engine runs lean and then releasing and converting it to harmless gases during a regeneration cycle. Because its condition directly affects NOx outcomes, the system must be visually inspected to ensure it’s present, properly mounted, and free of damage, leaks, or missing hardware. A visual check should confirm secure mounting, intact exteriors, no cracks or corrosion, no exhaust leaks around the canister, and proper shielding or insulation as needed.

Other after-treatment devices exist, but their primary roles aren’t NOx reduction. A diesel particulate filter targets particulates, a catalytic converter handles a broad mix of pollutants but not as the primary NOx control, and a diesel oxidation catalyst mainly oxidizes hydrocarbons and CO. The NOx adsorber is the component whose visual integrity is crucial for lowering NOx emissions.