What emissions-system repairs should I check before buying a diesel?

Ask for emissions-system history before you get serious about a newer diesel truck. Look for records related to EGR parts, DPF service or replacement, DEF system repairs, NOx sensors, exhaust temperature sensors, regen complaints, warning lights, and any recent dealer diagnostics. On inspection, make sure there are no active check-engine or emissions warnings, no limp-mode history, no fresh code clearing, and no vague explanation about deleted parts. Short-trip use and high idle time can be harder on emissions systems. A truck with documented repairs is not automatically bad; hidden or unresolved emissions problems are the bigger concern.