Determining whether your HVAC system requires a new blower motor or a blower resistor (commonly referred to as a speed control module or relay) depends on which fan speeds are functional. If your blower motor fails to run at any speed whatsoever, or if it emits a burning electrical odor and trips the circuit breaker instantly, the blower motor or control board is typically at fault. Conversely, if your blower motor works fine on high speed but refuses to operate on low or medium speeds, the issue is almost always a failing blower motor resistor or switch that fails to step down voltage correctly. You can troubleshoot by testing electrical continuity across the resistor terminals using a digital multimeter. You can verify the correct diagnosis when replacing the failed resistor or motor successfully restores all selectable fan speeds.
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