I have an older Bernina 1630 and have the same issue with the bobbin sensor. Sometimes it works and sometimes not - I think it is probably caused by the differnert types of threads that I use. I know that the sensor is a terrific tool - If it doesn't work I try cleaning up the bobbin area and if it comes on great if not I am afraid that if I poke around too much that things may go out of wack - I love working on this machine and because it is older I try to be cautious and hope for the best - really don't want to mess anything up with her so that I can continue to enjoy her.