You are entitled to a return of all replaced parts, except warranty and exchange parts, but you must request them in writing before performing any work. If you authorize work over the phone, the shop must keep the replaced parts and make them available when you pick up the vehicle. As a vehicle owner, you have the right to keep any part that is removed from your car during a service or repair, regardless of its condition. When booking your car for work, it's important that you notify the shop at this stage so they don't discard any old parts that have been removed from your vehicle.
Should you ask your repair shop to keep your old auto parts? Well, it might be a good idea if you're trying out a new store; it could be part of how you build trust in the store. It's not a good idea if you don't know what you're seeing. Now cars are very complicated, and showing someone an electronic sensor doesn't mean anything to them. I suspect that's why customers don't ask us to keep the parts.
We will continue the practice behind the scenes in case anyone asks. If a vehicle owner refuses to pay for repairs already borrowed, the auto shop has a legal right to keep the car.