The CEM is known for connection problems. You can sometimes clear the faults by cleaning the terminals, but this is often only a short term fix (if it fixes it at all).
It's worth removing the smaller rectangular connectors (blue,green,brown) on the CEM and checking inside for any signs of water ingress, or corrosion (white or green powdery deposits),
Clan is correct that the long term fix for communication issues is replacing the connector terminals. It's a fiddly job in an awkward position, but it's not much more complicated than soldering two wires together (for each of the 6 terminals that need to be replaced).
CEMs can fail, but the replacement needs to be programmed to your car, or the immobiliser will not allow the engine to start (among other things). This can be done with a brand new CEM by a volvo dealer, or your original CEM can be cloned into a second hand unit.
Most garages are useless when faced with this fault on these cars, as they only go on what their fault code reader says, and communication errors will trigger random unconnected faults to be stored, so it's not uncommon to hear of cars with many parts replaced and the original fault still present. When all it needs are half a dozen connector contacts that cost a few quid each.
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