Hi thanks for your replies but all I can say is that various zones trigger randomly at times sometimes months apart I’ve changed detectors every time a fault has occurred even tough resistance is fine detectors are always positioned away from the usual heat, draft and mains sources and I’ve sometimes decreased sensitivity of detectors but don’t like putting zones on double knock unless absolutely necessary we normally use texecom reflex on domestic jobs and have done for years without too many issues have even changed transformers and checked mains connections and in reply to al-yeti yes we have sometimes bought from electrical / security wholesales if we need something urgent and regarding the log unfortunately doesn’t seem to show any pattern or logic to the problems and as good as texecom tech support are they just tell you to go through the things already done then suggest change the board