The Mass Air Flow sensor measures the mass of air entering an engine. The fuel injection uses this and other information to predict the correct fuel/air mixture. The MAF sensor used on the Cadillac STS-V and several other GM vehicles is the ACDelco 213-4222, GM p/n 15865791.
This sensor is actually made by Hitachi. It is a combined sensor, and measures the mass air flow as well as the temperature of the air passing through the sensor.
This MAF is a card type MAF, which allows use of the same low-restrictive flow MAF sensor in a variety of intake diameter systems. The Camaro and the STS-V for example use the same sensor.
What the hu-bub is about is that some lots of the sensor are –while working correctly — less responsive, and some lots are more responsive. Although the part numbers are the same, the more responsive examples have a letter-number lot code along the side of the device, as opposed to a single-letter designator for the less responsive examples.
My STS-V MAF sensor is a “D” model, so I have ordered a cherry-picked letter-number model from Apex Motorsports to replace the current one. I hope to swap to the more responsive MAF next trip to the dyno.