From a post on BITOG. Any insights on validity? I use an UG and as far as I know my cars have had no serious side effects over hundreds of thousands of miles. I have posted this here before...just an FYI Before this year, I was all for things like this. I even went out and bought my own tool just for monitoring my vehicle while I was driving. I used it quite a bit, and still use it to pull codes, and check on general vehicle health. I used to have it plugged in ALL the time in my truck. Here is what I had. http://www.scangauge.com/ Now…I no longer drive with it plugged in unless I am actively trying to solve a problem. I was at a week long training course at the SAE headquarters in Detroit. I was asking some questions about vehicle networks and communication, and earned myself the opportunity to meet a very high up engineer at a foreign car company. He was specifically at the SAE headquarters to meet with the heads of several insurance companies in regards to the “passive” scanning devices that you plug into your OBDII port to get a discount. The short story is…anything plugged into that port disrupts vehicle communication. The OBDII port is just an interface into the vehicle network (it’s a node on the internal vehicle bus) – think of it like a gateway. Well, different nodes on a bus have different priority. That makes sure that important messages like vehicle speed, throttle position, AIRBAGS going off, etc. has a HIGH priority, and their messages can actually kick messages of lesser important off the bus. Well, the OBDII node is the HIGHEST priority on the bus because it is supposed to be used for diagnostic purposes (ie. just in the lab or garage setting). When you plug anything into the OBDII port, that device just became more important that the computer that controls vehicle stability, your airbags, your ABS, etc. ***** was meeting with the heads of these insurance companies to urge them to NOT offer those devices to the public because it has the chance to make their cars LESS safe and cause performance issues. In talking with him, the summary was “don’t leave things plugged into your OBDII port, or you risk messing up your car.” So…now I no longer drive around with my scanner plugged in.