If the system has already has had dye entered in it , ck the a/c drains with the ultraviolet light and see if it is the evaporator which is common. Most of the time you can smell it from the center vents. You can also read how many bar the pressure switch has via the pushbutton control unit .

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Mercedes Technician 7 Years (retired to Hyundai)
2000 Dodge Durango
98 Mazda truck