Sorry for the lack of response. I don't think there's many diesels on this forum.
If you brought it to the shop to fix the light and it being in limp mode, I'd say bring it back: they didn't fix it.
If you'd like to attack it yourself, start by finding out what codes it's throwing. You can replace the sensor (especially if it's not too spendy) as it may indeed be bad, but it's a shot in the dark.