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when checking a tstat r to c is 24 and with nothing calling r to g y w o will be 24v when you complete the connection to any terminal the circuit is complete and 0 across

Try reversing hi voltage to the unit , a class i attended claims polarity will cause flame sensor fault .

Is there a head master on this condenser ? If it bypasses the coil hot liquid enters the evap and no cooling

sounds like your outdoor unit is not producing heat . When your thermostat calls it starts your heatpump and elect heat strips and when you are about 2 to 3 degrees of set temp the strips drop out...

for ac only oversizing a coil 1/2 ton is ok for greater humidity control but not for heatpumps