I'm an old shade-tree mechanic -- my specialty is VW Beetles, the real
Beetle, not these new wannabe Beetles with water-cooled engines up front.
My experience with code sensors is zero.

As I understand it, newer vehicles have sensors all over the place that send
information to a central point where that info can be read by some kind of a
code reader.  On my wife's Nissan Maxima, I had to crawl upside-down under
the dash, pull back a piece of carpet, and count the flashes on a red and
green LED on the side of a computer box.  I guess the easiest way to do this
is to purchase a code reader for the specific vehicle, plug it in somewhere,
and read the codes.  And, I guess somewhere I can find a list of codes for
my vehicle and what they mean.

Having said all that, I have a 1997 Mazda B4000 pickup, 4.0L V6, 4WD,
5-speed, purchased used a couple of weeks ago.  Yesterday my ABS light came
on and will not go out, 4WD indicator lights flash, and the interior dome
light will not come on.  Dealer said it's a GEM module and they could tell
that from reading the codes.

What kind of code reader do I need to read these codes myself?  Where do a
get a list of codes and what they mean?  Can I screw up anything with my own
code reader?  What will the code reader tell me that I need to know?

Thanks.