Code Readers
ELM327 On Board Diagnostic (OBD) Wireless Adapter
The Super Mini ELM327 Bluetooth Adapter is an inexpensive and very easy way of getting OBD data.
It plugs into the OBDII Port just under the Steering Wheel.
Google Wireless Wifi Scanner OBD2 OBDII Diagnostic Reader for Android iPhone iPod.
There are many Apps available in the Apple iTunes Store or Android Google Play Store for connecting to the ELM327 via Bluetooth and can be used for many diagnostic tasks including reading Diagnostic Trouble Codes (DTC).
For example:
Here is a link to an article on using an ELM327 On Board Diagnostic (OBD) Wireless Adapter and a Samsung Galaxy S4 Smart Phone for automatically logging the engine coolant temperature versus time.