Please clarify:
Are you talking about the low HVAC fan position (which applies to all climate settings, not just A/C)?
Or are you talking about normal vs max A/C? This would obviously be for the A/C only.
If talking about the fan (left-most HVAC control), note if only high works or if there is only one speed that doesn't work. If only high works, that suggests a blower motor resistor. For any other issue, the HVAC control itself might have a problem.
Good luck.