external alarm remaining on after entering in code on a Horizon CP35?

Upon powering the Aritech Horizon CP35 alarm panel, both the internal buzzer & external buzzer go off. On entering the code only the internal alarm goes off and the display reads a 'flashing' 7 which according to the manual is a fault condition and to contact the supplier. Can you please let me know how to reset this fault?

Thanks

Joe
May 2012