I had a P0173 engine code causing the check engine light (CEL) to come on (see my post). So I bought my own OBD2 app for my iPhone and MacBook Pro. I believe it will also run on other smart phones and PCs including UNIX and Android phones. But for me the simplest and cheapest option was to buy an app for my iPhones 5 and 6 and iPad 3 from the Apple iTunes store. It was about $10 for the OBD Auto Doctor. Actually there are several available and some may be free but this is the one I chose because there is a version that runs on the MacBook and it seems to be comprehensive.

You also need some way to plug the software into the car's OBD2 socket which it typically located near the base of the steering column. Interestingly this socket is standard on virtually all cars to allow emissions testing and has been accepted my most manufacturers. It also appears that the error code numbers are also pretty much universal with the same codes used by all manufacturers and the apps include databases giving you a brief interpretation of each code number. There may be a few brand specific codes but they don't seem to be much of an issue.

Independently from the app, you can buy various plugs to fit the OBD2 socket in the car. I chose one that is a WiFi radio transmitter that links to the WiFi system on the smart phones and computers using standard internet technology. You can also get Bluetooth plugs and also USB cables to connect to computers.

I got the WiFi transmitter for $20 by searching eBay. It is an ELM type. It has no problem linking to the iPhone and does not require a password. That fact worries me a bit because it is theoretically possible for someone to log into you car computer and use it to unlock your car or reprogram it. So I would suggest unplugging it when you are not using it. Some models do require a standard password.

OK, so once you have downloaded the app and plugged in your WiFi transmitter you connected your WiFi link to the OBD2 computer using Settings on your iPhone. Then you are ready to go. The app connects and immediately displays any error codes. You may need to have your phone very close to the transmitter for it to work well.

There is a set of 3 horizontal lines at the top left corner which represents a menu list. Click on that and a menu of options comes up:

Summary
Trouble Codes
DTC Database (of codes)
Monitors
Sensors
Information
Settings
Help
About

If you click Trouble Codes it tells you what codes have been set like P0173.

The DTC Database gives you a brief description of the meaning of each code.

In the Trouble Code section there are tabs across the top that let you choose different types of codes and info:

Confirmed
Pending
Permanent
Freeze frame

Freeze Frame shows the readings on all the major engine sensors at the time the error occurred.

Sensors shows the readings on various sensors. If you click on one while the engine is running it will display a graph of the sensor reading against time. e.g. I found that my oxygen sensor voltages are oscillating up and down in a sine wave at about 2-5 seconds per cycle and I think this may be causing my P0173 error - see my separate post).

If you display an error you can reset all errors to clear the computer. As far as I recall you do this by clicking the icon consisting of a square box with an arrow pointing straight up from the center of the box. This gives several options. You can also send all the results to someone or yourself by email etc.

I found this to be a fantastic product and mechanics have told me it is just as good as their professional systems that cost $3000. Could that be true!