You need to go to the air handler and find where the thermostat wires enter and connects to the control unit in the air handler. Then note each thermostat wire color and its corresponding terminal designation at the air handler control unit. You should see designations at the air handler connection site (possibly a printed circit board) similar to the ones on the thermostat. If a silver wire connects to the R terminat at the air handler, then that silver wire needs to connect to the R terminal at the thermostat. I am assuming the same set of wires in the cable at the thermostat are the same set at the airhandler (ie no splice along the way). Hope that helps.