Miles on the odometer are the reason a diagnosis visit happens at all. When a warning light comes on, the shop often has to open the system before anyone can name the fault. That search time is billed as diagnosis, and it is sometimes billed before any repair is agreed.
What the contract may or may not pay for
Some contracts list the parts they cover and the labor for those parts. Others work by exclusion, covering everything except a stated list. Under either form, the diagnosis step is a separate question. A contract can pay for the repair while leaving the search for the fault to you, or it can name the search as part of the claim. Read the claim section for the words diagnosis, teardown, inspection, testing, and labor hours. If none of those appear, ask in writing.
Questions to put to the seller
Ask whether a diagnosis is paid under the contract, and if so, up to what point. Ask whether the shop must be one the contract approves before it starts testing. Ask whether a diagnosis that finds no covered fault is still paid. Ask who authorizes the teardown: you, the shop, or the company. Get each answer in the contract text, not only on the sales call.
What to check in the paper
Look for a diagnosis line in the covered-items section and a matching line in the excluded-items section. If the two disagree, ask which one controls the claim. Note any limit on labor hours, any requirement for a preapproved estimate, and any rule that the car must be at a particular shop before work begins. Those rules decide whether the diagnosis is a paid step or an out-of-pocket one.
Before the warning light
Ask the shop you already use what it charges to diagnose a fault and whether it will contact the contract company for approval before it opens the car up. Keep a copy of the estimate and the diagnosis note. A clear record makes any claim easier to review, whatever the contract covers. The front page lists the published terms of companies that accept cars at high mileage, so you can check each one against this list.