Likely ABS Module.
Located next to the ABS hydraulic pump.
Need Torx E5 socket to remove it:
http://www.modulemaster.com/volvoabsremoval.html

There are many advertising the fix for this (Rocha, Module Master, BBA, ebay etc and etc.) but I strongly recommend BBA for the following reasons:

- They are authorized BOSCH repair facility in the US
- The rebuilt ABS Module is warrantied for life.
- I replaced mine and a friend's ABS Module with units from BBA, so far so good.
- BBA is Inexpensive....2 options:
a. Remove your own unit, send it in for repair, they will ship it back, you install it.
Cost: $69.00 ABS Module + $20.00 shipping.

Pro: You save $30.00 (see below)
Con: you will be driving WITHOUT ABS Module, liability issue: NO ABS, No Speedometer.

b. Get a mirror and read off your existing ABS Module Part Number, Call BBA for a rebuilt Unit, they send you a rebuilt Unit, you ship them your defective Unit.
Cost: $99.00 ABS Module + $20.00 shipping.
Pro/Con: Opposite of the above.

HTH