The check engine light came on in my 97 850. The dealer reports that the mass flow sensor is bad and needs replacement. I can do this easily enough, but before I spring for the hardware, I would like to understand what might have gone wrong.

I would think that if the mass flow sensor failed, the engine would have no idea at all how much air was getting in to the intake manifold, and thus wouldn't be able to determine how much gas to inject, which in turn would kill the engine.
However, the engine runs fine, so I assume the failed sensor is just somehow unable to tweak the gas mix as needed for emisson control. If the change is that subtle, how does the diagnostic stuff even know what is wrong?