[Update] I pulled out my multimeter, found a video on how to test for continuity, found the manual for my multimeter to see how to do it using my equipment, then started testing wires. I found that of the living room window sensors had an open circuit. So, I climbed into the attic and started chasing down wires. I found them pretty quickly. They looked fine. No chew marks or anything to indicate damage. I followed them all the way to where they penetrate the wall and go to the security system box. No damage.

Totally perplexed, I decided to check the window again. The magnet had fallen off! I would have sworn I checked that last time! Anyway, I reattached the magnet to the window and everything is working fine now.

Thanks for the feedback!

[Original post]

One of the zones in our security system went offline. I opened the box and discovered they have six sensors wired in series with a resistor on that zone’s circuit. While I’m tempted to just disconnect the zone and start testing wires, I don’t want to fry anything. Any advise before I start mucking about? It’s Zone 4 in the pics and the resistor is wired in series with the red wire going to the ZCOM terminal 113. The green wire is on terminal 14, Zone 4.