Hello again I've recently come across a fault with the above type of detector. I seem to be getting either the sensor false firing or possible emc problems. I've lowered the sensitivity with the internal jumper but the fault is still present, all connections and continuity through cable seems to be good this is why I'm now thinking it could be emc. If it is emc does anyone know of a cure that doesn't involve re-running of cable. Thanks in advance for any suggestions.

I'd be surprised.

First of all if confirm its not environmental. Can you swap 2 units over ie between rooms?

If not if completly mask the lens and are if it continues. There are many more likely reasons before if suspect interference.