the blower motor resistor is bad or the connector going to it
The 2004 Chevrolet trailblazer air conditioning fan switch has three settings. The switch can malfunction, and only work in the high setting.
Had one that the fan motor was going bad Was burning out resistors with high draw
blower motor resistor
The fan control module in the engine compartment has gone bad.
If only high speed is effected check the high speed relay and/or the fan switch/electrical connections for problems.
the blower motor resistor is bad or the connector going to it
You need to replace your Blower relay,
try replacing the blower motor resistor
Bad resistor or a malfunction in the switch.
blower motor resistor needs to be replaced
Defective Heater Resistor Pack.
Check the low speed relay and the resistor at the motor