DUHH! I should have read this post more throroughly, it is about POST-CAT sensors.

It looks like the pre-cats are mounted on the exhaust manifold, #1:

I have not seen a DIY for these, but from what you say it should be straigtforward. I'm fairly fearless with engine things after successfully changing the oil filter assm. gaskes WITHOUT removing the fan, but don't ask my comments on BMW's torx socket belt tensioner--what a PITA.

I'm assuming that although BMW would like me to buy and change post-cat sensors as well, that only the pre-cats really need it at 100k. Gong to do plugs and trans fluid as well.

Bill