Alpine 105 Boiler - "Auto Reset High Limit" sensor error

My 3-4 year old Alpine 105 boiler is used for domestic HW and 2 radiant heat circuits. After it ran without issue for the first year, it has been plagued by problems since. At the beginning of each heating season (when the radiant is in use?) I've had some type of control failure:

The first year I was out of town and the service tech came and jumped out the LWCO - he said "it wasn't needed". (?)

The second year the "high limit sensor" was replaced. The unit seemed okay afterward.

Last year the high limit sensor was replaced again. The unit ran for a day or two and then the problem repeated itself. The tech came back and said the sensor he installed was bad and replaced it. A manufacturer's rep came with him, and also adjusted some of the software settings.

Yesterday afternoon I kicked on one of the radiant zones and went out for the day. Came back, and the unit is in error mode "boiler safety limit open". I jumped out the float switch, no change. Took the sensor replaced last time, and connected the 2 gray wires to it (and held the sensor in my hand - not installed). The boiler ran.

My guess is that the sensor is doing it's job, and sensing something that won't allow the boiler to run. Kicking on the radiant circuit makes me think it could be a low-water or air problem (or circulator problem?). Any ideas/suggestions on how to better diagnose the problem? Any suggestions on how to get some hot water until I can fix it?

This morning I pulled out my voltmeter and confirmed continuity across the two terminals of the high limit sensor that was removed last year. I performed the same check across the sensor that is still installed in the Alpine, and did not get continuity. Could it be simply that the sensor has failed? Can I swap out the one removed last year for the one currently installed?

To do this, I am assuming I can isolate the boiler, drain some water to remove the pressure, and simply remove the old sensor. A few turns of dope tape, install the new sensor, and open the feed water valve. Is there anything else I need to do? Will I need to purge the boiler? If yes, how do I do that? Any other ideas/suggestions? All help greatly appreciated. Thanks to all forum members for reviewing my post.