Yep, looks like a cable issue, you know that the power cores are OK, so you could take readings again, but use either the red or black to the greens and yellows in turn, this would tell you if just one core is broken or both. If just one then there is a possible way to bodge a working pir. The zone/alarm cores are basically a -ve coming out of the zone terminals and returning to the zone terminals. So you could, theoretically loop the -ve in the pir to one side of the alarm contact, and use a good core, yellow or green, to the other side of the alarm contact, and bring this into the relevant return terminal for that zone. No guarantees that this will work on your panel, I have done it in the past, depends on the panel, and it would be suck it and see as to which zone terminal it returns on, normally the right hand side one. Even more preferable is to run a new cable, but obviously I don't know how easy that would be to achieve. Right. I see. That makes sense. So, if (for eg) the yellow was fine (and green broken), keep the yellow in the PIR and at panel as it is, disconnect the green at the PIR/panel ends, and use a small piece of core to loop from the black core to the 'hole' (sorry) where the green previously was in the PIR and in the panel itself, sharing the black wire for both the -ve of the power and the alarm circuit. Are there any risks with this do you know? I'd rather not run new cable as it'd show up (would have to run it along the walls), but I suspect that's going to be needed. Btw, the power to one of affected sensors has now gone. Might be wrong but this might suggest some mouse or so activity (no recent building work done). Not sure if the fact that the wires are plastered over might cause them to decay. Thanks again for the tip.