They get these codes from the on board diagnostic systems on their cars. Cars built through 1995 have a diagnostic link under the hood, and the codes are retrievable by anyone with the knowledge to access them. 1996 - 1998 diagnostics require the use of a scan tool to retrieve the codes. 1999 and later cars require the use of a specialized PC called Vadis. Dealer only at this time. Also, the only service manual available from Volvo for 1999 and later cars is wiring diagrams. Everything else is accessed only on the Vadis PC.