I use the machine almost daily for garment sewing and quilting. I had the machine in the shop in early October and by the end of December the bobbin sensor code was once again showing on the machine.
My husband-the fabulous computer geek and can fix anything man-was in my studio and asked what was the problem. I removed the throat plate and opened the bobbin door and moved the shuttle case down to show the inter workings of the machine. He looked at the machine and discovered it has 2 sensors one magnetic and one possibility a photo sensor for the bobbin. He took a can of air and sprayed the sensors for quite a period of time. We put the machine back together and bingo the no bobbin sensor code. So I now blow the dust off the bobbin sensor every time I change the bobbin and no more problems. Who Knew?????
One more tidbit of information. The bobbins have silver-colored rectangles around the bobbin. If one of these is scratched or nicked the bobbin sensor will not see the bobbin and the bobbin code will appear on the screen with a question mark. Check your bobbins with a magnify glass to see any problems. Don't use these bobbins–I have put them in a bag and hope that some day Bernina might replace them, but I doubt it.
I love the 830 for all its bells and whistles and the wide machine bed for quilting is fabulous. Now that I have solved the bobbin problems; I love it even more.