Faulty blower motor resistor: The blower motor resistor regulates the speed of the blower fan. If it's faulty, it may not be able to provide power to the high speed setting. Check the resistor for any signs of damage or overheating, and replace it if necessary.
Blower motor failure: If the blower motor itself is faulty, it may not be able to run at high speed. Check the motor for any signs of damage or wear, and replace it if necessary.
Faulty wiring or connections: Check the wiring and connections between the blower switch, blower motor resistor, and blower motor for any loose, damaged, or corroded wires or connectors. Repair or replace any faulty wiring or connections as needed.
Faulty blower switch: If the blower switch itself is faulty, it may not be able to send power to the high speed setting. Check the switch for any signs of damage or malfunction, and replace it if necessary.
HVAC control module failure: In some cases, a faulty HVAC control module (also known as the climate control unit) can cause the blower switch to malfunction. Check the control module for any signs of damage or malfunction, and replace it if necessary.
