Turning DTCs off does not tell the PCM to ignore the functions, just not to trip the errors.

If you turn off all A4 DTCs and also the trannie A4 tests
the MM6 as to tuning requires little other then making the right changes to idle, timing and torque management.

Simply make your tuning changes then drive the car with a good PCM scanner and see if any other DTCs have to be turned off but a 1998 has fewer DTCs and functions then newer PCMs