Any help with my car problem? Doesn't engaged in D,1,2,3 position only on R.

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No messages, no light on on dashboard. It happened a day after when mechanic guy changed O2 sensors and erased the triangle light on dashboard.The only thing that I've done was driving from the service shop to my home and the next morning I had this problem. One more detail, when you put in first it engaged very week and you can go to D but has no power.I towed it to the mechanic and He told me that they scanned it again but no codes at all and He is sure that is transmission problem. I have to admit they are not Volvo guys. Thanks for spending time reading and understanding my problem, it will be a big help if someone has any experience.