2 pipe & 4 pipe sensors

[Rob2019]

Hello

The questions are regarding the Sensors used with the Thermostat in a Condominium building.
What are 2 pipe & 4 pipe sensors? What should be the typical resistances of each one?

 

[Rob2019]

It's Honeywell Thermostat model TB8575A1000 used in Condo building. This thermostat was working fine for the last 5 years. Presently, I don't see Cool option. Honeywell Tech support says it could be defective Pipe sensor. I am not sure if that's correct.
However I would like to know What should be the typical Ohmic resistances of 2 or 4 pipe sensors, 10K, 20K?

By the way, what are 2 pipe & 4 pipe sensors?

 

[Avo Mk8]

It's Honeywell Thermostat model TB8575A1000 used in Condo building. This thermostat was working fine for the last 5 years. Presently, I don't see Cool option. Honeywell Tech support says it could be defective Pipe sensor. I am not sure if that's correct.
However I would like to know What should be the typical Ohmic resistances of 2 or 4 pipe sensors, 10K, 20K?

By the way, what are 2 pipe & 4 pipe sensors?

The part number you give doesn't seem to be a sensor (eg. like a thermistor), the literature available in the UK says it is a complete thermostat assembly providing switching. Their description says:
"TB6575A, TB6575B, and TB8575A are a family of Digital Fan-Coil thermostats, that provide line/low voltage on/off control for various fan-coil units in residential and commercial applications. Great for use in hotels, condos, and school classrooms"
They appear to be used in the air handling units, but I don't know any details about 2 and 4 pipe, apart from drawing the obvious conclusion!
Presumably being an electronic device, the fact that yours has worked fine for 5 years doesn't mean it will continue to work for ever. It is now obsolete according to the Honeywell website, perhaps implying the system you have is older than that?