At our shop an RO with a “Return for Prior Service” line is created and a copy of the previous RO is printed. The vehicle is given back to the tech who worked on it. This vehicle pretty much has priority over any other vehicles being worked on. If it is found to actually be a problem with the previous repair or an original misdiagnosis the vehicle just gets fixed. If it is a new unrelated concern the customer is given an estimate for repair. in most cases the shop eats the diagnosis time. The shop owner handles all of these cases after the initial call from the customer reporting the concern.