My 2005 Impala has a problem. The fan blower motor can no longer be controlled by the fan control knob and the fan keeps running when I turn the car ignition off.

The blower motor relay is energized when the Blower Motor Control is set to the High position. This removes the blower motor resistors from the circuit and battery voltage is then applied directly from the HVAC BLO fuse (RH IP Accessory Wiring Junction Block) to the blower motor though the relay switch contacts. The blower motor then runs at high speed.

HVAC Blower Controls Circuit Description
The blower motor is a variable speed motor. The motor operates at a higher rate when voltage is increased to the maximum level.
The blower motor resistors reduce the voltage supplied from the A/C FAN fuse (LH IP Accessory Wiring Junction Block) when the blower control is set at low and medium speeds (speeds 1-4). The blower motor relay is energized when the Blower Motor Control is set to the High position. This removes the blower motor resistors from the circuit and battery voltage is then applied directly from the HVAC BLO fuse (RH IP Accessory Wiring Junction Block) to the blower motor though the relay switch contacts. The blower motor then runs at high speed.

fan motor relay is for fan speeds
heater and AC

blower motor has a resistor
did the motor bench test ok
if fan motorand resistor are ok
i suspect
a faulty on /off switch in the dash panel

i think that system has electronic directional flap actuators not vacuum ..if vacuum might be a leak ..

I would say its the blower switch inside the control module.
I doubt turning the switch on and off would affect the blower motor but you could check the motor with a meter to find out.

read this post on what's causing blower motor to run all the time.

http://ricksfreeautorepairadvice.com/blower-motor-wont-shut-off/
Blower motor wont shut off Ricks Free Auto Repair Advice

Your BCM (body control module), must have gone bonkers. Have it checked out. Might just need a little tweaking-like re-setting.
Good luck.

Your impala doesn't have a resistor , it has a blower motor control processor and it doesn't work the way you think it does
Chevy Trailblazer EXT Blower Motor Won Turn Off
Blower Motor Control Processor
The blower motor control processor is an interface between the HVAC control module and the blower motor. The blower motor speed control, battery positive voltage and ground circuits enable the control processor to operate. The HVAC control module provides a PWM signal to the control processor in order to command the blower motor speed. The processor supplies 12 volts to the blower motor through the blower motor voltage supply circuit. The control processor uses the blower motor ground as a low side control to adjust the blower motor speed.

HVAC Blower Motor Circuit

the resistor to the blower motor is bad or the motor itself, the resistor should be right next to the blower fan a heavy gauge wire plugs into it, or it might even just be the wire, it is common for them to melt

to stop the guessing ,replace blower motor and blower resister/if motor is bad it can make the resister go out,u may have a blower relay also thats bad replace if the control speed switch has not got a burned spot on the back where the wires plug up then this should repair ur car/it sounds like a lot to replace and maybe u think unnessasary but these componts work all together and affect one another

The purple wire voltage, black wire is ground, at the blower motor. At the blower motor control processor, the red wire is a fuse protected circuit, hot all the time. With hvac off, if nothing on the speed control circuit, doesn't the problem have to be the control processor? Do you see it differently? Just my opinion.

the blower motor resistor is damaged probably in the process of removing the heater core replace the blower motor resistor, and please dont forget to rate me!!