Losing fan speed settings, is the most common complaint of a bad blower resistor.
On this 2001 Chevrolet Silverado  the customer has lost fan speeds 1 and 2.

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On the 1999-2007 General Motor Truck Series the blower resistor is located near the blower motor, under the glove box.  To gain access to the blower resistor you have to remove a cover box, held on by two 7mm bolts.

On this vehicle the customer has a aftermarket floor mat making it difficult to remove cover, so we will work around it. Normally this cover would be removed.  The blower resistor is mounted by two 5.5mm bolts, with two wiring connectors.

One is a 2-wire pigtail coming off the resistor going to the blower motor. This one is hard to reach, use a small screw driver to release the connector tab.  The other is a 7-wire connector from the A/C control head. This connector should be inspected for damage, it is very common to find a melted connection here that would require the pigtail connector to be replaced also.

To install, first connect the wiring.  But, before completing the reassembly, test the system for correct fan speed operation.

*Tech Tip- If your finding yourself replacing the blower resistor for the second time, most-likely the blower motor is a fault. As the blower motor wears, it is very common for the motor to demand more amperage.  This demand of more amps often is too much for the resistor, causing the resistor to fail prematurely. Check APDTY.com for the Blower Motor for Your Vehicle.