K, so to get the obd2 to work it's quite an extensive process unless you want to spend like $200. I have an iPhone 4S and I use DashCommand. In order to be able to do this for about $15 (cost of the Bluetooth OBDII sensor on eBay) you'll need to first JailBreak your iPad. If you have the iPad 3 on 6.0.1 you can't do it yet, but if you have something older, or you haven't updated to iOS 6.0 yet then you're in luck (jailbreak for iOS 6 on A5X chip devices should be available around Christmas).
Okay now, once it's jail broken here are the steps to follow.
1. Go into cydia and add the sources cydia.hackulo.us
2. Install installous and roqyBT4 and the roqyBT4 Bluetooth OBDII add on (you need this because apple products won't connect to bluetooth OBDII devices, only wifi ones and those are about $150).
3. Get a Bluetooth OBDII from eBay.
4. Download the app DashCommand free from installous ($50 in the app store).
5. Connect your OBDII device to your OBDII port in your car and turn your car to stage 2.
6. Open roqyBT4 and search for your device. When it connects make note of the "data port" numbers that come up when it says it's connected.
7. Now open up DashCommand and go to settings/connection/OBDII data port and enter the numbers you got from roqyBT4.
8. Now it should be connected and you can click "dashboards" in DashCommand and it'll give you all your cars info, diagnose, etc.
Note: Every time you want to use it you'll have to go in to roqyBT4 and re connect to your Bluetooth OBDII. DashCommand will automatically connect to it from now on as long as your phone is connected to it, but your phone won't automatically connect to it. If you don't know how to jailbreak, use YouTube.
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