XC60 alarm issue but not the siren?

[surfslammer]

Hi, any advice on this one please.
XC60 R Design AWD 2014

I have the alarm service required message appearing constantly with every start now. It was previously intermittent. Having read a number of posts, it seemed the usual siren unit would be the likely suspect. To be fair, the siren unit was corroded, and I have swapped it for a second-hand replacement which appears to be working.
I still have the error message.

I have the following observations:
• A friend (electronics whizz) has checked and tested connections and voltages etc. and all seems to work fine on the replacement unit.
• The remote key sets the alarm with the indicators flashing, the mirrors closing and the red alarm light on the dash flashing all as expected.
• The remote key disables the alarm with all the expected indicator flash etc.
• There are no false alarms triggered.
• When using the remote to lock the car/set the alarm, then opening with the blade key, the alarm triggers and the siren blares out as expected.
• When using the remote to lock the car/set the alarm but remaining inside the car, the alarm is not triggered by movement (the reduced movement guard was not enabled).
• The movement sensor on the roof interior seems to be firmly in place and no visible issues.
• There are no obvious other issues occurring.


I’m no tecchy but wondered if any of you guys have any pointers as to what the likely issue is and what other checks I can do?

Any advice much appreciated.