They same way. It only takes one visit to fix a car. Even with a computer and codes. You still have to use the same tools and diagnostic path. Verify and duplicate the customer concern. Inspect the vehicle and check all systems. Battery, charging, fuses, etc. You still have to look up service information, bulletins, wiring diagrams. They were in a book, not on a computer but still the same stuff. Scan tools don't fix cars, just like oscilloscops didn't fix cars back in the day. A diagnostic path and a good technician fixes cars.