Two problems related to side/window air bag have been reported for the 2008 Subaru Impreza. The most recently reported issues are listed below. Please also check out the statistics and reliability analysis of the 2008 Subaru Impreza based on all problems reported for the 2008 Impreza.
My 24-year-old daughter, who is 5'2" and weighs approximately 100 pounds, was sitting in the passenger seat of my 2008 Subaru Impreza for the first time when we discovered that the passenger occupancy sensor does not register her weight and shuts off the passenger-side airbag. By moving around and adding weight to the seat we were able to turn the airbag back on temporarily -- but when she is just sitting in the seat as she normally would, her weight is not high enough to trigger the sensor. I called the Subaru dealer, who told me that there is no way to change the calibration of the weight sensor. The sensor cutoff is unreasonably high; there are many adults who do not meet this weight requirement (a google search turned up many similar experiences with Subarus). This miscalibration could have dangerous -- even fatal -- repercussions.
Passenger side airbag goes on and off, when my 18 year old daughter sits in seat! she's 5'2" 130lbs. Took to dealer and they had her ride in two Impreza's off the lot and they all did it!.