The AC after a while looses its coolness and blows hot air

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The evaporator temperature was measured at 13.5c front and 16.5c rear, Refrigerant pressure at 11 bar, with coolent temp at 87c. The MB service told me initially everything is fine but on taking a second and third time they now say the compressor is faulty while the reading above are the same all the time. The compressor kicks in on switching the AC on. Can I relay on the MB Service and change the Compressor? Please help