Pushed by EPA, new car's computers are wired into every part of the car. When you take the car in for an inspection sticker, they don't measure the emissions anymore, they plug into the car's on board computers and ask the car if it's burning clean. If the microprocessor thinks the car is clean you get a sticker. If the microprocessor thinks the car smokes to much, no sticker.
When your car's microprocessor give you thumbs down, fixing it can be tough. The mechanic asks the microprocessor what's wrong. The microprocessor replies with a bunch of code numbers. You have to have a code book to figure out what's what.
The car companies only make the full code book available to their dealers. Independent mechanics are left looking at a bunch of numbers. Nothing they can do without the code book, which the car companies won't give them. Gotta take it to the dealer. As a car owner, you know that taking the car into the dealer is gonna cost you heavily.
The independent mechanics are supporting a "Right To Repair" law that would force the car companies to publish the full code book. Car companies and their dealers (dealers can be a potent political force) are dead set against it.
Me, I take my car to a good independent mechanic, and he takes good care of it. I think he, and all the other independent mechanics should have access to all the codes. My car will run better after Bob Warden fixes it than it will after any dealer mechanic works on it. And for less money.