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Hello I have a Trane XV95 and AC XR15 and a Nest Thermostat  (I know I will replace it soon).

When I start my AC unit this year it will not shutoff.  I checked the actual unit and the electrical part that magnetizes and connects.  It works fine.

I tested the voltage on the furnace and the thermostat.  For some reason when I test the Y and C wires I am getting 17v.  When I test W and C I get 0v.  Which is what I expect because the thermostat is off.  When I test Rn and C I get 24v which is expected too. I get the same results if I have the AC unit breaker off and perform the same test.

My question is when I test the Y and C wires should I be seeing a 0 value like the W?   If this is correct then I think it the base of the thermostat that is broken.

Note, I also tested using a different wire and I get the same results.  Any thoughts would be helpful , thank you.

Y and C = 24V when the thermostat is calling for cooling.

Y and C = 0V when the thermostat is off.

Did you try disconnecting the 24V wires from the outdoor unit?  Is the contactor staying closed even when the thermostat is turned off?

Given the fact you get 24V between R and C that would rule out a broken C wire from the thermostat, so likely the thermostat or contactor is the problem.  Or maybe there's an issue with the C wire going to the outdoor unit.

Thanks for the ideas. 

I checked the contactor in the AC unit and it is not stuck. It turns off when I shut off the AC unit breaker.  I did disconnect the 24V wires on the furnace coming from the outdoor unit and it still reads 17V. I am thinking it is the thermostat.

When the thermostat is off the contactor on the outdoor unit is still staying closed.  I assume it is because there is 17V on the line.

I guess my next test would be a new thermostat and try it out.

What I would do is turn the power to furnace. Remove the front of tstat. Take wires out of the tstat and confirm there is no continuity between each one.

Connect R and Y  together.  Tightly enough so they don't come apart, but in a way that I could pull them apart neatly with my fingers without letting the wires touch anything else.

Turn the power to the furnace on. Confirm the AC turns on.  Take the wires apart, confirm the AC is off.   

If AC remains on. The problem is likely related to the unit outside, or furnace board terminals related somehow.

If AC turns on and off properly. The problem is likely in the tstat.

If continuity shows between wires that shouldn't be. The issue is likely with wiring somewhere.