I recently spoke to a contractor that told me that when he checked his limit switch, the resistance was "good".  I asked him "what does good mean?, what was your ohm reading?" He seemed a little caught off guard but then continued on, describing how he connected his meter to the switch and it showed that it was closed and gave him a beeping noise. "Ahhhh, you were checking continuity and not necessarily resistance." There is quite the difference between the two, we had a little discussion on it, and how they related to the low voltage circuit of the equipment he was working on.

Continuity or "ringing it out" is a quick test, somewhat reliable but not very detailed if you will. A resistance check/test, is a bit more in-depth of a test, and tells us more about the integrity of the circuit or switch. I'm going to do my best to explain the difference, and how they matter.

Picture two bodies of water, like two large lakes, between those two lakes is a creek. That creek, would be considered the "continuity" between the two lakes, meaning that they are connected. Now picture those same two lakes being connected by a river. A river being larger than a creek, allows a larger and less restrictive connection or "continuity" between the lakes. The river would allow more water to flow between the lakes easily, where a creek (more restrictive) may flow much less.

The same theory applies to electrical wires and switches, if the resistance  increases (dirty contacts, too small of wire, too many devices in the circuit ect.) then the ability to flow the proper amount of electricity is reduced, and it may not be "good" enough to do what we need it to do.   

Low voltage HVAC control circuits are typically protected with a 3amp fuse, and will generally operate at around 1 amp or so (not a-lot of power). A resistive/restrictive thermostat, limit switch or add on safety switch can easily reduce the ability for the signal to come through clean, and provide proper operation. 

Happy testing and good luck out there!