Mercedes Vito 112cdi Question/Problem

The problem I'm having is this: 88,000kms.... When you put it into D, 1, 2 or 3, after about 3 seconds, the Check Gearbox lights up, and according to the book, the transmission will only work in 3rd
gear. That's the whack you feel I think, and the fact you can't really rev it much.
You can turn off ignition, the light is gone...everything's fine...then put it in D and it's back! Can anyone enlighten me as to what causes this and how it can easily be rectified?
And is there anything I can do with this sealed transmission?
Thanks LJ
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