Alarm sounding on 2004 Volvo S60 D5 for no apparent reason.
Temporarily, you can go to the fuse box inside your car [open the driver's door and the panel is on the end of the dash, which is hidden when the door is closed.] And pull the last fuse, #38, for the siren.
The owner's manual states that pulling the fuse, or if it's damaged/burnt/blown, that the alarm will sound. That's not true. BTW, I learned of all this from three different Volvo forums/user groups, tonight. My '03 S60 started having a near constant running of the alarm. [It's usual 25 second run, a very short pause, and then it'd go again.]
Pulling the fuse disables the siren, but the rest of the alarm still works as usual. Blinking lights, and if it has it, an ignition kill. You can verify buy closing all doors, but keeping the drivers window down. Then arm the alarm. Wait 30 seconds. Manually pull up on the door lock pin. Then open the door. The amber lights should start flashing. And your dash will have a message that the alarm was triggered, and to check the vehicle.
The problem and it's permanent solution: the batteries in the siren module go bad after some time, and if they leak their electrolyte fluid, that'll corrode the circuit board and/or quite commonly, it turns out, corrode one of the two +/- wires connecting the battery to the circuit board. Thus, messing up the alarms sense of reality, and triggering the alarm at random times/intervals.
Need to get a replacement siren module [can't get circuit board separately, apparently.] and either have fun trying to replace it yourself [dealing with rivets] or have the dealership do it. They'll say you need to do a software reset, but you just need to arm/disarm the system three times to reset it. [according to the forum posts I've read.]
Here are links to what I read:
on Jun 11, 2019