What questions should I ask before buying a used diesel truck?

Start with questions that reveal history, not opinions. Ask how long they owned it, why they are selling, whether the title is clean, what maintenance records are available, what it was used for, what it towed, whether it has been tuned or deleted, what repairs were done recently, what problems still need attention, and whether there are active warning lights. Ask for cold-start video, VIN, photos of the underside, tire date codes, hitch setup, door-jamb payload sticker, and any service invoices. A good seller can answer directly. Vague answers, missing records, fresh code clearing, or pressure to buy quickly are reasons to slow down.