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When checking transfluid level on A-class a special dipstick must be used, which must be bought separately. If the level is not correct- transmission will not operate normally. So please make sure first of all, that the fluid level is precise. If needed, i can geto you instructions for this. Of course it is crucial that the fluid is of good quality and filter is not blocked. I suggest to replace the fluid and filter as a first step. It will also give you a picture, if any serious mechanical damage is present by metal shavings in the fluid.
Next step would involve transmission electronics check and reset with Star Diagnosis computer, to reset self-adaptations of the transmission control unit. It might be possible that one of the electrohydraulic valves in the tramsmission is broken. Once this is done, it will be possible to determine if any real mechanical damage is present.
If no electrical faults are detected- transmission will have to be overhauled. It is not uncommon for the the gear clutch cylinders to crack and leak, causing slippage of the respective gear.