I have an A6 Quattro 1999. The A/C unit runs but does not produce cold air. Some times it give hot air with temp in the 90's

Please give me suggestions

You could be out of a charge and need to get it recharged. That is the simplest thing to do. Other thing you can check is to see if the compressor comes on.

of note is, when A/C is swiched on the compressor comes on, the related pipe gets normally cold, but the with climate control set at low the blower continues to blow warm to hot air. also unpredictably from time to time thesystem would function normally. i wonder if this caris equiped with some cabinet temp sensor which may be malfunctioning