alarm will not set - veritas 8c?

Have tried everything in manual under trouble shooting. Alarm will not set have done all e.g the walk test, strobe test etc. etc. But when we try to set it always highlights zone 3 and beeps 9 times. We have link tested the detector in zone 3 and does not appear to be a fault but the red sensor light does not come on in zone 3. Have tried programming to omit zone three (but will not work) and when we try to set alarm it still lights zone three up on control panel and beeps 9 times. (9 beeps according to manual says problem with tamper switch, we have removed control panel cover and the cover on sensor three and have stretched springs on tamper button as per manual for these to reset but still can not set alarm- anyone have any idea's?? Everything else works fine when we test thanks