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Onstar has NO air bag impact sensors. Those sensors are for the sensing and diagnostic module (SDM). It is the SDM that signals via the class 2 circuit that the bags have been deployed. Technically, the signal goes through the BCM, the master, but it essentially comes from the SDM to the Onstar module.
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As for air bag sensors, there are two in the front, two in the side and a roll over sensor (if equipped with side air curtains), seat sensor (presense and position) and the SDM.
For a frontal collison, primarily the SDM detects a repaid deceleration, and then determines what force to blow the bags depending on the front sensors. If no one is in the passenger seat it will not blow the passenger bag. For side collisions the sensors signal the SDM to blow the side bags, but the front bags may not blow.
The SDM used on this vehicle is primarily the same used for years, even for those that had NO frontal sensors.
There are NO sensors in the rear. The signal to Onstar may tell them front bags, side bags or all, but that is about all the info the module can send.