It's a bobbin spring and it does have to be put in the right direction and the hole at the bottom of the bobbin case needs to be clear.

From the manual:
Insert the bobbin case brake spring into the bobbin
case as shown in the drawing, ensuring that the four
brake cams (thin arrows) engage into the bobbin case
notches (see arrows)..

You know what, Marianne? It just might be that as I remember that when I cleaned everything and re-placed the bobbin, I did notice that the washer seemed to only be half way in. . . I'll check that too! Thanks!

Have you checked the bobbin washer inside the bobbincase? Mine got misaligned once and it drove me bonkers that the machine went off again and again. Eventually I removed the washer and brushed out underneath that too and when putting it back realised that it had not been in all the way.
Hope you find a solution.
BTW it's not really a washer but can't for the life of me remember right now what it is called - late at night here now and lost an hour last night due to Summertime begin.
Marianne

Thanks, Susan, I will try the "shutting off the alert" route if one more cleaning doesn't work. . . it should be sparkling with the number of times I've tried that. I do clean it when I change the bobbin, usually every couple of days that I sit down to sew. Something has to work. . . even if I have to take it in!

Here's a quote from another 640 owner

"Yes, you can turn it off in the "Display Messages and Monitoring Functions" of
the setup screen. Getting there is outlined in pages 97-98 of the manual.
Basically you click Setup > Select 5 for Audio Settings then go to the next
page and turn off the lower thread control.

Before doing that though, I'd suggest a thorough cleaning of the bobbin case,
the race, feed dogs and sensor on the bobbin door. When I got my 640, I got a
lot of false alerts and read many online messages online complaining about the
bobbin sensor. I was leaning toward turning off the but I decided I that
before doing that, I would try to faithfully brush away all the lint in the
bobbin area, the bobbin case, and feed dogs every time I changed the bobbin -- I
haven't had a false alert in months."

I hope this helps.

Hey, Susan, My bobbin door was shut, but it still goes off about every 7 seconds. I'm going to work on it again today and then take the rest of the day off.
Happy Easter!

The problem is, on some of the Berninas, leaving the door open makes the sensor go off every 15 seconds.

Thanks again, I'll try both of these suggestions as soon as I get some coffee in me! Tossing and turning last night about this. . . . silly!

I just did a search for "sensor" in the 640 manual.
It's very possible that the sensor isn't activated in the set up program. The information on this part is on page 92 of your manual.
Make sure the bobbin door is fully closed.
I'm not an advocate of doing this however it might work until you can get it to the dealer: leave the door open while you're stitching. The sensor won't be active.

Happier quilting!
Teri

Check your owner's manual. It's just a setting somewhere. If no one else answers and you can't figure it out, I can ask on the Bernina Yahoo group.

Thank you everyone, I guess I will take it back unless I can figure out how to turn off the bobbin sensor light! Any ideas about that, Susan? Thank goodness, this quilt doesn't have to be finished for a bit.

In the meantime, until you get it back to repair, you can probably turn the sensor off. I have a 730 that has never had a functioning sensor. Make sure you shut the bobbin case door.

When I firt got my 640 I kept getting the same sensor error. It turned out that the cable to the sensor area had a loose connection. The tech connected it correctly and it's worked fine since. Fi

Because the 640 is an embroidery machine there is a sensor that reads the bobbin that I suspect is malfunctioning.
Take it back to the dealer. Make an appointment so you can see the tech and show the tech what's happening. That will give them the best information.
My suspicion is that there's something interfering with the sensor causing it to "misread" the bobbin.