Michael, I believe your car has the OBD-ii engine management system. You can confirm this by looking under the steering wheel for a plug that will be open (ie, no mating plug, just the female end of the connector). Should be just under the edge of the dash. If you have this, then you have the OBD-ii system. With this system, if you are just looking for a basic code reader, then stop by the local auto parts store and they have one for like $70 (can normally be "rented" from the parts store for free and that will clear the code). But, please keep in mind that the code will not clear if the fault condition still exists.
If you are looking for something that you can monitor engine parameters and have more options, then you are looking at more than likely getting one of the cables that you buy online. These normally cost about $150 and will include the associated program. This will require you to use either a PDA of some sort or a laptop. But, the information that can get from a device like this is very impressive.
If you don't have the plug that I am talking about, then you have the old EEC-IV system. There is a plug in the engine bay for plugging in a code reader. These are like $20 or so at a parts store. You can read the codes from these systems using an analog multimeter, but you need to connect the multimeter to 2 specific pins. I don't know what they are off the top of my head.
Chris "Thermo" Coleman and Jackie, the 12 XJ