There are two possible locations for the diagnostic ports on an Audi S2. The image here on the left shows them arrowed on the left hand side of
the main fuse box in the scuttle area. The blue terminal (#1) is not used.
The white terminal (#2) is for the K and L communication lines to the
various electrical modules within the car. The black terminal (#3)
provides 12v and ground supplies.
The alternative location is in the drivers side (UK RHD models) footwell
beside the auxiliary relay panel. They may be tied up out the way so you
may have to search for them.
The purpose of the on board diagnostics on the Audi S2, is for communication with the electronic control unit (ECU). If the ECU detects a fault,it will store a fault code. The diagnostics allows the user to interrogate the ECU to determine which, if any, fault codes are stored within the ECU. If your car is in the dealership for a service, one of the instructions within the service schedule is to check the ECU for any stored fault codes.