Check the control module

Fig 1: Control Module

Courtesy of VOLVO CARS OF NORTH AMERICA.

Check:

that the control module is of the correct type (this depends on whether the car is equipped with STC or not)
that none of the pins or sockets on the connector are damaged
that there are no contaminants on the mating surfaces between the control module and the ABS hydraulic modulator
that no part of the control module seals remain on the ABS hydraulic modulator valves.