Replace the blower motor resistor. High speed working normally indicates the blower motor is usually fine, so both parts rarely need replacement. An open circuit in the resistor was confirmed and replacement restored all speeds.
A 1999 Chevy Prism with about 49,000 miles had AC and heat blower operation only in the high position. The expert said the solid-state blower resistor commonly fails from moisture and corrosion, and that a working high speed is a good sign the blower motor is okay. The owner replaced the resistor, found an open circuit, and reported that all speeds worked again.
