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Hi, I'm Brian. I'm here to help!The most common cause for this is not ensuring there is a jumper on the thermostat between R (or Rh) and Rc. The two R terminals MUST be jumpered together where you connected the red wire. You can confirm if this is the problem by setting the thermostat fan switch to ON (instead of Auto). If the fan doesn't run in with the thermostat fan switch in ON then chances are you need to jumper as stated above.Other than that, I would need the make and model of the thermostat, furnace and outdoor unit and we would need to check all wiring and programming. It's also possible you have a problem with the outdoor unit if the blower runs but the outdoor unit doesn't.