Diagnosing a faulty blower motor
If you suspect your motor is bad, you should check to make sure before you replace it. Begin diagnosis by shutting off the power to the furnace. Then, try to spin the blower by hand. If it will not spin freely, check to see if it has oil ports on the blower. If it has oil ports, you should add a few drops of oil to see if it helps the motor to spin freely. If there are no ports or it still does not spin freely after adding oil, the motor should be replaced. If the blower spins freely, feel the motor to see if it is hot. If the motor is hot to the touch, allow it to cool. Check all the wire connections to ensure they are tight, check the run capacitor and replace it as necessary. Then you can check to see if the motor works properly. Turn the power back on and turn the fan on at the thermostat. If the blower does not start or fails to get up to full speed, shut the power back off to the unit and continue with furnace blower replacement.