My app is working quite well now. I am implementing some OBD II communication through CAN messages, following Wikipedia Article on OBD II (https://en.wikipedia.org/wiki/OBD-II_PIDs).
If I've time I will publish the code during this weekend.
I used this library (Cordova plugin) for BLE: https://github.com/randdusing/BluetoothLE
@jamesatfish, serial communication service UUID is 7a1fb359-735c-4983-8082-bdd7674c74d2
I subscribed to its characteristic b0d6c9fe-e38a-4d31-9272-b8b3e93d8657 (with indicate method), and send commands to its other characteristic b0d6c9fe-e38a-4d31-9272-b8b3e93d8658 (with notify method).
But they are interchangeable. You can also use the same characteristic to write and receive data (subscribe).
Here is the source code for my app: https://github.com/eiannone/CBT-Diagnostic
To ease dealing with OBD protocol I implemented mask filtering on firmware for CAN messages id. That's because ODB II CAN messages id are only in the range from 0x7E8 to 0x7EF. You can find it in my <a href="https://github.com/eiannone/CANBus-Triple/tree/can-mask">v0.5.1 can-mask branch</a>.
I suggest also to disable bluetooth rate limiting, in order to avoid missing some CAN packets when you receive quick responses from the car.
The apk is here: https://github.com/eiannone/CBT-Diagnostic/releases/download/0.1alpha/android-debug.apk