What engine do you have?
I have a late 1995 UK 850 2.5l 10V with the Fenix 5.2 engine management.

On my model the old two blocks for error code reading which were in the engine bay have disappeared, and there is now an OBD-II socket. However the populated pin positions don't tie in with OBD-II specs.

What we surmise has happened:
In the US all new cars had to have an OBD-II socket from 1996.
In 1995 Volvo knew they were replacing the 850 with the V70 within a year so a major redsign of the electrics wouldn't be financially viable.
Therefore they got rid of the engine bay code reader, and just ran the wires to the OBD-II socket, and that's why an OBD-II reader doesn't get any codes.

For my model there is a simple circuit you can make up consisting of three fly leads, a resistor, an LED, and a switch which allows you to read the codes from the OBD-II socket. It all depends which pins on your OBD-II socket are polulated.....we have the UK version figured out, but I don't know if the US version is the same.

If you want, I could email you the circuit diagram...it's use is exactly the same as for the old fly leads in the engine bay