Randmaster

Once again, I received the error message " Passenger Restraint System Failure!!!" on my 2004 530i. I already took the car to my stealer once and they told me that they couldn't find a problem with anything and they reset the hard code. They noticed in the memory that there was a low battery warning that came on a few times so they THINK it might be a battery problem even though the battery is fairly new. I told them about the recall for the passenger seat mat but they claim that wasn't the problem. Mmmm.
Nevertheless, I'd like to be able to read/ and reset the codes myself with a diagnostic tool. Does anyone have any recommendations for this code reader? There are tons of different brands out there.
Any suggestions on what I should get?

Thanks all!
Randy