I have had my Scantronic 500r+ for 18 yrs but it no longer recognises its pirs. I have put into engineering mode and tried to reintroduce them (done before many times when renewing batteries) but the control unit doesn’t recognise any of them and fails to give the beep to indicate it’s learnt them. Everything else on the control unit seems to be working fine. I tried a factory reset. All pirs have brand new batteries and fuses in control unit are fine. Anyone any ideas what it might be?