Shut the power off to your furnace. Get a volt meter and head outside to your a/c condensing unit. Shut off the disconnect and gain access to the electrical. Remove the two wires going to the contactor coil and seperate them so they arent touching each other. Now head to the thermostat, set it to cool, and turn the power back on to your furnace. Head back outside, take your voltmeter and check for voltage across the two wires you removed. You should have 24v. If you do then your problem is in the outside unit, most likely a bad coil on your contactor. If it does not have 24v, then work your way inside. Go to the furnace with your voltmeter, check between terminals Y and C for 24 volts. If you have it there then your problem is in the wire from the furnace to the outdoor unit. If you don't have it then it is a communication problem between the furnace/air handler and the thermostat. Just make sure the wire going from the furnace to the outdoor unit isn't connected to R and Y, it should be connected to Y and C at the furnace. Happy hunting!