A/C works on high speed only

This is a common complaint when the blower motor resistor is bad. http://repairpal.com/heater-blower-motor-resistor

Repaired same prob on various vehicles. It's eigther your blower control module or possible a blower resistor. The module is your blower control in your dash easy to replace the panel pops out an the control is screwed into the dash. Bad thing is its a dealer item not cheap between $80 - $100.