I am at a Volvo dealership where I had my check engine light diagnosed (I wasn't confident in my assessment after checking the onboard code). They've told me that I need a new O2 sensor, which needs to be replaced. Can anyone tell me where they have found a good price on these sensors?? I've looked at fcpgroton.com and they carry Walkers and Bosch- which of these or other brands would you suggest? Finally, I am wondering how I can tell if I have a "California Model" or not? Thank you VERY much for your help. Trying to decide whether or not I'm going to attempt to replace this part, feeling a little apprehensive...