sigalabs.com have designed and build a great OBD2 Arduino Shield using STN110 IC :
The STN110 is a multiprotocol OBD to UART interpreter IC. It provides an easy means of accessing vehicle data, including diagnostic trouble codes, MIL status, VIN, Inspection and Maintenance (I/M) information, In-use Performance Tracking (IPT), and hundreds of real-time parameters.
It is compatible with famous ELM327 commands but introduce many new great features.
Vehicle OBD2 Arduino Shield with STN1110 IC – [Link]
Any car newer than 1995 has a fairly sophisticated computer used to monitor various systems ranging from accelerator pedal position to catalyst efficiency. This is a brief introduction to one example of the hardware used to diagnose automotive systems.
A look inside an OBD2 tool – [Link]
On PC desktops the RS-232 serial ports are about to disappear from most computers (especially from laptops), replaced by the USB connection. This project was begun as a response to build simple ELM327 compatible USB adapter for On Board Diagnostic (OBD2) monitoring.
OBD2 ELM327 compatible AllPro adapter with USB – [Link]