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Research blower motor resistor and you will find many pictures and a how to procedure.
Blower motor resistor.
It is the blower resistor. It happened to mine and from what I can tell it is a common problem. Here is a site that helped me it also shows you how to replace it. You can find the part at AM Auto parts. http://www.sparkys-answers.com/2009/01/2002-gmc-envoy-no-low-blower-speeds.html
the resistor for the blower motor is busted blower works on high with out it
Check the blower motor resistor as it controls the fan speeds. Also check the blower motor as it will burn out resistors if its drawing to much amperage
Bad blower resistor?
The blower motor works on only one speed. The blower motor resitor pack is defective and needs to be replaced. It is located on the blower housing in the engine compartment just below the blower motor.
Probably your blower motor speed resister is burned out.
There is a resistor near the blower that is bad, known cause is high speed only.
If you are saying the Blower Motor only works on high, then your problem is a defective Blower Motor Resistor Pack. Replace it.
Blower switch and connection had a melt down.
blower motor resistor needs to be replaced
Defective blower motor resistor pack.