Sounds like I have the same problem as you have, except mine has failed to disarm completely.
From what I understand, the receiver (or transponder?) is attached to the siren apparently making removal easier than fixing! Does anyone know how to remove the immobilser (and alarm system) without calling in the main dealership and paying them an arm and a leg? The car isn't worth much, but other than the immobilser problem, its 100%. The car is sitting immobile in the Crystal Palace area of London! Is there any volvo mobile mechanic who can disentangle the immobilser? I can pay a reasonable amount for an hour or two of your time! Call me on 07723044953