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At the blower motor, back probe and ground the Blue/Black (BL/BK) wire and verify the blower motor runs on high speed. If the blower works when grounded, ground the BL/BK wire at the blower resistor connector. If the blower does not run when grounding the wire there, locate and repair the open circuit between the blower motor and resistor. If the blower runs on high when grounding the wire at the blower resistor, turn the blower switch to the high position. Then jump the BL/BK wire at the blower resistor connector to the Blue/White (BL/W) wire at the resistor connector and the blower should run on high speed. If it does, replace the faulty blower resistor.