Hi, oodly, welcome to the forum. I just tonight stumbled across this thread by accident. I don't check the forum every day, but when I do, I look for questions pertaining to alarm systems in the thread titles. Which is to say that I don't usually look at the first post of every one of the dozens of threads started on this forum almost every day.
So for future reference, it would speed the process if you would mention "home alarm" in your thread title. A lot of guys know a LOT more electronics than I do, but I'm an alarm technician with some decades of experience and usually alarm system troubleshooting has more to do with particulars of how various alarm systems work than arcane electronic knowledge.
To start with, it always helps to name the make and model of your alarm control panel. For example, Honeywell/Ademco Vista-20P panel, GE Concord, ITI SX-V, etc, etc. Even if the problem doesn't hinge on the type of panel (which this probably doesn't), it lets a pro know how to tell you to test or try certain things. Different panels work different ways.
Also: Don't be secretive about the zone you're having a problem with. It takes no more time to type "Zone 1" than to type "one of my zones". Not all zones are alike in some panels, believe it or not, Please name which number zone you're having trouble with.
Now you didn't mention having a problem with your system, but I'm guessing that that's why you were metering the zones in the first place. It sounds like a pretty straightforward problem. The 2.2K End-of-Line Resistor (EOLR) is wired in series with the magnetic switch in/on (which is it?) the door frame. The mag switch is a NO switch that is held closed by a magnet on/in the door when the door is closed and brings the magnet in proximity to the switch in the frame.
So when the door is closed, the switch is closed, completing the zone circuit (aka zone loop), and the panel terminals "see" 2.2K resistance, which typically pulls the voltage down from about 12.5VDC to about 5.5VDC. (The exact readings vary from one model to another, and sometimes on different zones, and are a red herring to chase unless you have some kind of arcane problem.)
When the door opens, it opens the circuit and the voltage goes back up to ~12.5VDC.
From what you've described, the mystery zone ('cause I don't know the number yet) loop/circuit is reading open or high-resistance (In alarm jargon, we call it a "high-resistance open") with the door closed, when it should be seeing only 2.2KOhms.
The most common causes for this symptom are, in order of statistical likelihood, are:
(1) Door magnet is missing, too far away from mag switch/contact, or misaligned. (most often)
(2) Loose, corroded or severed connection(s) at mag switch or control panel terminals. Sometimes it's as simple as switch lead wires being twisted together and not soldered. Sometimes a crimp-on connection was crimped improperly and took years to go bad.
(3) Mag switch in/on door is bad. (much less often).
(4) Bad or corroded splice or severed wire the walls between panel and switch. (Very rare)
In some instances, the door switch is a plunger-type, where a button is pushed into the switch by a closing door; sometimes on the hinge side and sometimes a roller/ball -type plunger on the latch side. If you have one of these switches, this would be the time to describe it. They tend to be high-maintenance.
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A final word, in case you decide to order a new switch for your door; In the alarm industry, nomenclature for sensor switches is different than standard electronic nomenclature. "D/w" (door/window) switches are nearly always referred to as "Normally Closed" when they are in their "standby" state, i.e., with the door or window closed, which means with the paired magnet in proximity. What an alarm tech calls NC and NO in door/window mag switches is opposite what an electronics tech would call them. So be aware of that if you order new ones.