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Check between Y and C and G and C at air handler instead of R and Y and R and G.
You will get any reading between R and G or R and Y as they are the same "leg" so to speak.Thermostat is like a light switch breaking the R leg from transformer to all components.
WE will go from there.
OK sounds like thermostat is doing its job. We just are not getting the voltage outside or we have a high voltage issue. Go to outdoor unit and remove service panel. Check for the 27.5 volts at contactor coil while thermostat is calling for COOL.
If we have the 27 volts at contactor coil and plunger is pulled down, check high voltage in and out of contactor.
We can go from here.
OK look in main drain line and see if perhaps there is a float switch or if you have an aux drain pan. look for a float switch on pan and lets see if we can find what is breaking the circuit to outdoor unit.