OBDII Diagnostic Solution for $64 (+shipping)
There is a lot to write regarding OBDII diagnostics, but for those who are looking for an extremely reasonable priced solution here it is.
This solution uses the ELM interface (as shown on ScanTool.net). You can probably actually purchase the parts to build your own (roughly $55 for all the parts) and build it yourself if you prefer.
You can purchase a single protocol interface from ScanTool.net for around $50-$60. It comes with the interface and cables you will need to begin OBDII diagnostics (you may need a USB to Serial adapter as well). If you live outside of the US, check the ScanTool.net distributor list to save money on shipping and import taxes. This hardware can be used by a few different freeware software applications.
We offer a freeware application ourselves called Digimoto Lite (but you can use whatever software you prefer). The benefit of using hardware from a company like ScanTool.net is that it is open and many software applications operate with it. Other similar companies are Ozen Elektronik (http://www.ozenelektronik.com/) and Drew-Tech Technologies (http://www.drewtech.com/) ~ these are all hardware manufacturers who offer hardware options that are supported by various software applications.
If you use a solution like the ELM 320,322 or 323 you will need to determine the protocol it uses (ISO, PWM or VPW). Some interfaces (like the mOByDic 2600 from Ozen Elektronik) combine all protocols into a single interface, but it will cost more money. In the end, if you have the money, your better option is to purchase a tool based on a chip like the mOByDic 2600 (which we sell). Compared to the ELM327 based interfaces, it offers the same protocol support, but runs 400%-1000% faster depending on the vehicle and the protocol being used.
In 2008 all vehicles will be required to support the new CAN interface and some newer cars being released utilize this interface now. In general, import vehicles utilize the ISO interface, GM uses the VPW interface and Ford uses the PWM interface.
If you use a solution such as ELM or mOByDic + Digimoto Lite you will have the ability to log your sensor readings, graph them, view sensor data in guage format, view error codes and reset the MIL, freeze frame data etc.. You can also use the ELM or mOByDic interfaces to build your own custom software if you prefer. The hardware manufacturers I have listed above are very friendly towards third party developers interested in taking on such tasks.
Also ~ as a tip. If you are interested in purchasing any retail diagnostic solutions my suggestion is always to check eBay as well first. Many businesses post daily auctions and offer you the ability to save a ton of money simply by knowing where to look. My company no longer offers our products in the eBay market, but it is a good thing to remember anyway.
On a side note, we have always offered a discount code for all mp3car users. The current code we have available is mp3car15 (15% off any products through our site).