it should (IIRC) flash the led a number of times or beep a different number of times than normal whenu disarm the alarm, this will indicate what triggered it,

2 favourites are ultrasonics , and the bonnet pin switch, as these are at the front, they geta ll the rain/spray and salt in them and fail veryt often. , is it a 966 or 993 (2:1 upgrade) model? i thinki have an old handbook in mt draws somwhere