The little Bluetooth plug in is called an ELM327 i picked it up of eBay a few months back for about £6 delivered then the full app ( Ian Hawkins Tourqe pro) cost £2.95 that then gives you the capability of reading and clearing codes as well as a heap of useful live data for tracking down problems.
Here's a few screen shots:
On the phone opening screen with menu

In live data mode with on screen dials

My phone is the Asus Padfone 2 and with the handset I can slot it into the Padstation a 10.1" tablet and run it in tablet mode, here's how it looks on there:
Opening screen

Live data dials (you can customise them as you like)

You can run 7 screens and just flick between them here's one of both my upstream O2 sensors (bank 1 sensor 1 and bank 2 sensor 1) working perfectly in graph mode

Anyway just thought I'd share my thoughts and for the money its an awesome bit of kit, i use lots of diagnostic gear at work in my garage and this is so simple to use any one could use it.