What should I check before buying a used 6.7 Cummins?

Ask for maintenance records first: oil, fuel filters, coolant, transmission service, and any emissions-system repairs. A clean 6.7 Cummins listing should also make mileage, idle time, towing use, and title history easy to understand. On inspection, check cold starts, blow-by, turbo noise, exhaust aftertreatment lights, coolant loss, oil leaks, front-end wear, brake condition, and transmission behavior under load. For 2019-2020 trucks, verify high-pressure fuel pump recall history before you get serious. If the truck was tuned, deleted, or heavily worked, price it as a higher-risk buy unless the seller can show clean supporting documentation and the truck fits your local emissions rules.