The number one complaint vehicle owners have is bringing their car or truck to a repair shop with the fear of being taken advantage of. CodeKey is the solution to this fear. CodeKey is a sophisticated but simple, easy to use device designed to unlock the mystery of why your vehicles "Check Engine" light is illuminated. With CodeKey, you can be your own "vehicle doctor" and diagnose what is wrong with it. You may not always be able to fix it yourself, but you will know what part or system of your vehicle is not working properly, allowing you to make a more educated decision on who should repair it
More expensive, but pretty cool is the ScanGauge II. Nice toy that you can program to show things like fuel consumption, cost-per-mile, coolant temperature, engine speed, horsepower, and much more in real time. Not all at the same time though.
As a cheaper alternative get the Autel MaxiScan MS300 CAN OBD-II Scan Tool
This scan tool also retrieves generic and manufacturer specific Diagnostic Trouble Codes (DTC) and can reset the check engine light. The product description also mentions that it can read out the readiness monitors, something that CodeKey doesn’t do.
Autel MaxiTrip TP100 4-In-1 Auto Computer:
It does trip computer real-time information monitoring including average trip MPG, tank MPG,
average speed, average speed per tank of gas, miles to empty, and miles driven on tank and
serves as well as a scan tool. Scan tool functionality including reading and clearing trouble
codes on OBD-II/EOBD compliant vehicles,
freeze frame data and the ability to turn off check engine light.
The reviews are somewhat mixed though.