You have first to try to unlock the Honeywell Pro series thermostat with the default password "1234." Step1: Turn off the thermostat and remove the battery from it. Step 2: Then wait for a few 5-10 seconds. Replace your battery and turn on your thermostat. Step 3: You can check with the default password of "1234" if it unlocks. If this default password does not help in unlocking, you can take steps to unlock the Honeywell Pro series thermostat without a pin. Step 1: Check the 4 digit code of your Honeywell Thermostat. Step 2: You can note down that date code and add it with the default password (if the date code is 1550, add the default code 1234 to it, and your final password will be 2784). Enter this final password, and you will unlock your thermostat.