Could be as simple as a counter in the SW overflowing.

11 or 12 days is just over 255, the maximum count for an 8 bit (one byte) counter.

Thanks. "a counter in the SW": what would it be counting (hours?), and can it be disabled? I guess it can't be re-programmed to 10 or 16 bits?

If not, I guess I'm gonna have to send someone over after the 9th day and disarm and re-arm the system? Thanks for you help.

The only way your are going to be able to solve this, other than the shotgun approach of just changing parts, is to consistently replicate the problem while you are home. It may not take 11-12 days to appear.

Can zone-4 be left on and the others zones disabled while you are home?

Have you measured the controller end of the NC contact wiring with an ohm meter, at the controller end of the wiring, and compared it to the wiring of the other sensors? (They may be a resistor in line at the sensor, so it may not be zero.)

Have you tried relocating the sensor on the wall in case there may be something behind the sensor causing the intermittent trigger?

Thanks for replying. Unfortunately, zone 4 is the master bedroom and home office, so I can't leave that zone disabled while in the house. I could swap it with zone 5, but I have to go through it to do my laundry, so I can't leave it armed for 12 days consecutively.

About relocating the sensors, I know there's nothing electrical or AC lines behind them, and I guess magnets wouldn't affect them?

I had someone to whiggle the wires while I checked for continuity/short-circuitting at the control panel and everything was fine, but I will measure impedance of both zones 4 and 5 this weekend, as these two zones both have two motion detectors.

Maybe I am completely off the track, but it seems to me that "intermittent triggering" that only happens on the 12th consecutive day of being armed is probably not intermittent but rather something that is bound to happen after 12 days. I have never had any false triggering except those 4 events. I have gone on week-long trips, and never had an issue.

Brickeyee, you suggested "a counter in the SW overflowing"; I suppose you meant the software in the control panel board? I have downloaded the service manual, and I can't find any reference to a counter. What would be the purpose of such a counter?

The DSC PC1500 seems to be a popular system, because my Internet search led me to many discussions about it, and I personally know at least three persons who have or had them. Now that I think of it, my uncle is one of these persons, and him too has a false trigger every time he goes away for a long period of time, so the theory of the counter overflowing starts to make sense to me. Are we the only ones leaving our system armed for two weeks while I go on vacation?

Sorry for the long post, and thanks for helping me with my problem.

At this point, I wouldn't start swapping zones right now - it may just add more confusion.

As far as relocating, the sensors can be sensitive to RF interference and, although they shouldn't be, temperature changes on the wall.

But first, if you have the installation/service manual - not the owners manual - you can see if zone-4, or all the zones, are "supervised alarm zones/circuits."

When I mentioned it may contain a resistor (end of line), it is because of supervised circuits. Supervised circuits, in simple terms, the alarm control panel is acting in a tri-state mode.

It it always looking to see a certain resistance across the circuit; let's just say 1k, it's a NC circuit, and the resistor is in series. A cut of the wiring triggers a high resistance error and sets the alarm mode. Somebody who places a jumper across the wiring, triggers a low resistance error and sets the alarm mode as well.

The resistor value that must be used will be listed in the installation manual. Since ambient temperature affects resistor values, placement on a wall heated by the sun could cause enough change on a high tolerance resistor to cause an error - but that's just in theory and there's usually enough tolerance in the system, but it could happen.

A simple test to help determine sensor, wiring, or controller fault would be to first determine if this is a supervised circuit and it's resistance value. Disconnect zone-4 wiring, at the controller, and either jumper with the appropriate resistor (or just a short wire if non-supervised) and disable all zones but 4. Let it run for a couple weeks and see what happens. Just kind of flowchart logic from there.

But actually, I also have another thought here. You mentioned zone-4 is master bedroom and home office, so I'm thinking a PC in the home office... maybe a UPS power supply? Maybe one with an "auto test feature?" I even had an APC unit that by default ran an auto test mode "about every two weeks" - now with the full cycling of the UPS, and it's temperature changes, that could possibly trigger an IR detector... just a thought.

Thanks a lot for the reply. You've got a good point about the UPS, but I don't have one, and I unplug the computer and its peripherals from the wall while I go away.

While temperature variations are still a valid argument, I was gonna write that at least two false triggerings happened at around 5:00 AM, when the temperature inside and outside the house is at its lowest. Interestingly enough, 5:00 AM is also the time that I usually leave home for my trip and arm my system.

I'm gonna check the manual carefully to see what resistor value is needed, and I will install it in the panel downstairs. I know for a fact the only zone that has a resistor (1K, 1/4 watt, 10% tolerance) is zone 6, which is a fire zone, unless the detectors have them built-in.

Once again thank you, and I will keep you updated once I check if the proper resistors have been used.