Fault on an 2-wire Haes surveyor twin wire

I have a really strange fault on a Haes Twin wire Fire alarm system Sav wire / series 60 heads, when I activate zone 2 a fault led on zone 1 and detector remove LED Illuminates and common fault when the sounders are going off when they stop the fault led disappears . When I activate zone 1 or press the sound alarm button it’s ok no fault LED is Illuminated ‘ I have removed each smoke head and sounder one by one on zone 1 to test if one of the smoke heads is the cause of the fault, I am running out of ideas I have also checked for earth fault but looks ok. I did find an detectors with water damage but I have removed the device from the Circuit. Any ideas anyone?