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Cleaning Disk Worked For Me
Many thanks to the OP!! Cleaning the optical disk worked for me. Several minor comments to add...
On step 3, removing airbag: Keep the tool in the plane of the steering wheel and push straight in at 4 o'clock and 8 o'clock.
On step 8, splitting the steering wheel cowling: It's pretty easy if you pry in the right place. Don't pry on the sides near the split between the upper and lower cowling. Use a plastic tool to pry the upper cowling upward closer to the center of the upper cowling. To remove the lower cowling, put gloves on and use your thumbs to pull outward on the tops of the two clips.
Because it's unlikely that two distinct failures would occur simultaneously, if your optical disk has an obvious issue, it's unlikely that you also have a solder issue.
I found it very easy to clean the gunk off the optical disk with 70% isopropyl alcohol and a Q-tip. Scrubbing wasn't necessary; just a very light touch with the Q-tip. I didn't need to remove the disk or the IR waveguide (the clear plastic piece that sits above the disk). I rotated the disk so the portion I was cleaning was away from the waveguide and PC board. After the gunk was gone, about 20% of the optical issues remained; looking through the disk, the image was slightly distorted but it wasn't occluded like it was when the gunk was present. The top surface of the disk appeared to be smooth under magnification so I decided not to go after it more aggressively with a plastic polish. The 80% optical solution was enough to get my car steering angle sensor working again.
Because this problem shows up over time, it must be the result of some progressive condition. Therefore, it's likely that the problem will reoccur eventually so save a link to this thread.
And finally, one major comment: Shame on BMW for not addressing this serious safety issue free of charge. This optical disk issue should've been a safety recall. I love my car but I've lost a lot of respect for BMW on this one.
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