Plug Your iPhone into Your Vehicle's Onboard Diagnostic Computer
mlive.com
mlive.com
For reference, take a look at the leading OBD-II iPhone app Rev (http://www.devtoaster.com/products/rev/). I've actually owned and enjoyed this app for the past 2 years. It pretty much does everything. Anything you can do through the OBD-II port, you can do, and customize completely to how you need it. It also does the CEL lights, which is pretty standard. But selling to auto enthusiasts, this app focuses on the performance features.
No car geek would ever recommend GoPoint over Rev, but for someone that's interested in environmental impact and fuel economy, GoPoint's simplicity and price clearly wins.