The climate control system can fail or perform erratically due to internal problems with the climate control button electrical contacts. Replacement of the climate control assembly is commonly required to correct this problem.
Car Problem Reports
Mercedes-Benz S420 Wear in Climate Control Unit Buttons May Cause Climate Control System Failure
Mercedes-Benz S420 Problem
Visitor, 1996 Mercedes-Benz S420, 119,000 mi
Wear of buttn to increase temperature on driver side,wear of button to increase/decreaser fan and wear on button to select upper/middle/lower vents