Here's a breakdown of why this happens:
* Blower Motor Resistor Failure: The resistor itself can burn out, overheat, or become corroded. This prevents the lower voltage signals from reaching the motor, leaving only the high speed (full power) option functional.
* Wiring Issues: Wires leading to or from the resistor can become disconnected, frayed, or corroded. This can disrupt the signal, preventing lower speeds from being selected.
* Blower Motor Issues (Less Likely): While less common, a failing blower motor itself *could* contribute if it's only capable of operating at high speed. This is usually accompanied by other symptoms like unusual noise or the motor not working at all.
What to do: You should check the blower motor resistor first. It's usually located somewhere behind the glove box or under the dashboard, though the exact location varies by year. You can find diagrams and instructions online specific to your Impala's year and model. Replacing the resistor is a relatively straightforward repair.
If replacing the resistor doesn't solve the problem, then you'll need to investigate the wiring carefully. A qualified mechanic can diagnose and repair wiring issues effectively.
Don't hesitate to consult your Impala's repair manual or a reliable online resource for your specific model year. This will give you diagrams and precise location information to help with troubleshooting and repair.
