Gardtec 500 Alarm Fault?

I currently have a Gardtec 500 alarm installed which is now at least 10 years old. This morning the alarm started sounding and when looking at the panel it displayed a letter E which in the manual advises a fault with the entry/exit zone. The panel was completely unresponsive to any codes entered so I proceeded to follow the factory reset procedure by disconnecting the mains supply and battery then removing the pcb from next to the anti tamper switch. Once powered back on the alarm was in the same state, letter E displayed on the display. The external siren eventually switched off by itself but the internal panel makes more of a loud buzzing noise then a siren noise. I have currently left the panel with the mains and battery disconnected as this is the only way to stop it making the loud buzzing noise. I assume the unit is completely dead ? I am going to try and remove the sensors for the Entry/Exit zones but I assume this isn't the true cause ?