The part(s) or condition(s) listed below for the symptom Furnace blower runs all the time are ordered from most likely to least likely to occur. Check or test each item, starting with the items at the top of the page.
The control board has relays that regulate the voltage to each component in the furnace. If the relay that controls the blower motor fails while it is closed, it will send continuous voltage to the blower motor, causing the motor to run continuously. If the control board is at fault, replace it.
The wall thermostat has electrical contacts that control the power supply to the furnace. If the contact that controls the blower motor sticks closed, it will send continuous voltage to the blower motor, causing the motor to run continuously. To determine if the thermostat is defective, use a multi-meter to test it for continuity. If the wall thermostat shows continuity between the (R) and (G) terminals when the thermostat is in the off position the thermostat has failed and will need to be replaced.