Here's an interesting story regarding a recent experience with the air mass meter in my 1992 900S: For about the last year, I experienced intermittent Check Engine Lights (maybe once a month or two, at first). The frequency gradually increased, until I was seeing the C.E.L. pretty much every day. At the same time, the car's idle became rougher and rougher, and it nearly stalled a few times. All of the stored error codes had something to do with the mixture being too lean (e.g. bad mixture, lean mixture, oxygen sensor, etc).
I suspected it might be the AMM. I read a post in this group that suggested that unplugging the AMM could be diagnostic (i.e. if unplugging it improves performace, the AMM must be the problem). And so I unplugged the AMM, and quickly discovered that the car won't start with the AMM unplugged. Disappointed, I plugged it back in and drove off.
To my amazement, the Check Engine Light and drivability problems were totally cured, and have remained so. I don't know why it worked, maybe the plug was gradually coming loose from the AMM? Anyway, this was the cheapest, fastest, and easiest car repair job I have ever done.
John