Modern sensors should have 6 wires.
The only 2-wire sensors I can think of are some B&Q types which are dedicated to their systems.
I guess the fault will be a damaged wire or faulty sensor.
Can you omit each zone and set the system and see if you can find the problem that way?
Once you can identify which zone is giving the problem, you're halfway to finding the problem!
There is a "log" facility on the Paragon panel. Can you check the history to find which zone is triggering?
HTH
Inglishg
March 2010