Before rushing and speneding money you don't have to, there are two modifications you MAY require which could sort your problem.

Firstly the 1996 model and slightly later had a wiring crimp issue. There are 2, for some unknown reason, pairs of crimps which are the main feeds from the ABS sensors under the fusebox. If your car is in that class of year, check the fusebox for a green sticker on it. This will mean yours has had the crimps removed and the wires soldered and re-terminated safely. At the factory they were poorly crimped catching the outer insulation and causing intermittent connections causing speed sensor DTCs and extrapolation errors.

Secondly, there's a mod kit from Volvo to replace the wiring at the ABS sensors to the wiring loom. These are also known to be an issue. There are details of these in Vadis.
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Tek