Electronic Alarms and Home Security Devices - Zone No Longer Responding (Always Open)

So the history is that I bought a home with an ADT system already in place, it was installed when the house was being built. Thanks to this site I successfully am able to use my alarm system and it communicates with my cell phone via text. I have a concord express with 5 zones in use.

Everything has been working great since I got it running in January however for the past month Zone 5 has been reporting that it's open. This zone has 5 normally closed magnetic sensors, 4 on windows and 1 on a door. I ran a sensor test last night and found that all of the sensors in this zone do not respond when I attempt to trigger them.

I fear that a wire is broken somewhere in the circuit but I don't have much experience with this kind of troubleshooting. Does anyone have any pointers in terms of finding the fault? I was hoping it's something simple like the 2k Ohm Resistor going bad on the end of the circuit but I don't know how to validate this. Could I just disconnect the zone and do a continuity test?

One thing I did manage to measure is that all of the working zones are showing 4 volts running through them with a 0.5 volt dropoff after the resistor. However zone 5 seems to be running 0.5 volts higher than the other 4 zones. I did this test with everything connected and running... and I may have done it wrong so I can't vouch for the accuracy.

The first question: Did they run an individual wire from each opening, or is there only a single pair coming back to the zone input? If the former, it's pretty easy to isolate which specific opening is the problem, you simply test each pair and see which one is open.

They ran each pair of wires from the point of installation back to the panel and then joined them at the install panel.

The loops are unlabeled and it's a mess in there but they're all in there.

Good. All you have to do is disconnect the wires from the zone terminals and use the continuity test function of your meter to isolate the open pair. Once you do that, the way I usually test them is to jumper the open connection closed (either by removing it from the series circuit "daisy chain" or by stripping enough wire to put a jumper clip on the connection.) Re connect the zone and it should show closed. Then you turn on the chime function, and open and close the windows that are supposed to be on that zone one by one until you find the one that's not working.

The odds are that it's either a bad contact or simply a missing magnet (you did check for missing magnets, didn't you?).

Haha yes, I checked the magnets before diving into the wiring. I hoped that I would find one fallen from where it was mounted but all were in place. That being said what I didn't check is the exposed wiring on some of them... as it's possible that opening and closing the windows might have cut something.

I'll check into that tonight and perform the continuity tests as you describe and post what I find. Thanks for the helpful replies!

Thanks for the troubleshooting info. I used the methods you recommended and tracked down the open sensor... then tested the sensor and found it functional.

After spending the better half of today in the ceiling crawlspace I found that a rat had chewed a bout 1.5 inches of one of the wires completely out... leaving one strand fully intact. We bought a foreclosure and had to have an exterminator come out after we heard the buggars chewing on stuff in the ceiling a few weeks back. At least it's fixed, thanks again.

Beer 4U2

Ick, worst case scenario. Don't know why, but rodents seem to find PVC wire insulation just irresistable, and the harder the spot is to get to, the better...