[classicman]

I own a Nissan and there is NO READER Needed. It is a stand alone system. The computer is located under the dash by the console. Turn the ONLY screw all the way to the right, wait a few seconds and then turn it back to the left.

This activates the system and the "check engine" light will flash accordingly. It will flash rapidly (short burst) then slowly (long) this lets you know what the code is. For example three short flashes and then 6 long flashes means a code of 36. *note* there may be multiple codes and the codes may have 3 digits instead of 2. REMEMBER to reset the computer when you are done. All the instructions to access the on-board computer are also in the Haynes or Chilton manual
Take that code and look it up online or in the Haynes or Chilton manual (which EVERY car owner should have). It will tell you what the problem is and typically how to correct it. I have done this many times.

Again, I prefaced this with I own a Nissan - I'm not sure how this relates to your vehicle because of the make/model where it was manufactured... and so on.