Just had the same fault on my V70. I spent £80 on the icarsoft scanner to read the codes.
After clearing them I swapped the sensor wiring plugs from one to the other, if the code changes from sensor 1 to sensor 2 you'll know the sensors is the fault. If not then its possibly a abs ecu fault. I got my sensor from
http://www.volvobreakers.co.uk/volvo...vo-abs-sensor/
You'll need a deep 27 mm deep socket to remove the sensor. Its a bit awkward to get at but straight forward enough to do.
So far so good no warning message.