replace evaporator temp sensor after verifying fault code . it IS possible to read codes from A/C pushbutton display inside vehicle.But any MB shop with late model diagnosis capability can look at the 'actual values' in A/C ECU.. I would make sure shop disables menu item 'coolant (freon) monitoring' as this is a already taken care of by ElS (ignition switch ECU) and this caused a software conflict on early S classes . If evap sensor is bad it will read some inplausible number like -40C or similar if it reads ambient temp do NOT pay to have it replaced as a software issue is more than likely the cause. Some VERY early S clases.(early 99 production date) had evaporator tubes that were kinked during assembly but almost all of these were repaired under warranty as per TSB.. also make sure system is full of freon!!