Even in McDonalds if what they give you is defective they will replace it without question once you bring it to their attention.
If it turned out the door locks on the car were defective you'd expect them to be replaced under warranty. If the warranty had expired the situation would, admittedly, be a bit murkier - but you could still make a case that since the locks had always been faulty they'd be the manufacturer's responsibility.
Someone I used to work with had a car a few years ago on which the battery would mysteriously drain for no obvious reason. It turned out to be a defect in the infotainment system's firmware - and he was furious that he was expected to pay for the firmware update to fix it. (The car was long out of warranty, though.)