Hook up a scan tool, and read the code and see what the problem is.  Then
reset it after it's been corrected using that scan tool.

Check the computer for codes.  If you have a light then there is a problem.
These trucks are fairly bullet proof, check the code and fix it.  probably
something simple...  You can get a "reader" for a few dollars at Wal-Mart.
All it does is short two leads on the plug under the dash above the parking
break handle but the enclosed book is really what you are paying for...
Once you do short the two leads, turn on the key w/o starting the engine and
count the blinks.  You will get two sets of indicators fast blinks and
pauses.  The fast blinks count as numbers and the pauses as acknowledgement
to move on to the next number.  For example, with the engine off you will
get a code 13 (engine not running) it will blink once, wait, then blink 3
times quickly, then wait and blink once, wait and then three times quick it
will do this three times (13, 13, 13).  It will blink each code three times
then wait and will go onto the next code.  I bet you will get codes for
something like O2 sensor or temp sensor in your case.  You write down these
codes making sure to look though them at least twice to make sure you have
it correct.  You will know you have looped back when you get the 13 code
again.  Then look in the book to see what circuits they match.  Post back
once you have the codes.  The reader is not too expensive and should be
something you have in your tool box.  These trucks run for ever and you can
actually fix just about anything on them yourself with commonly available
tools.  You can also get a steel key that will do the same thing as the
reader but may not come with the book.  This is a Franklin fix (requires you
to buy something).  The 13 code is normal and should be ignored and keep an
eye on the wheel wells for rust and the under cross member that the tailgate
bolts to, this also rusts away quickly.  Otherwise just take good care of it
and it will take care of you.

Mark

p.s. LMC Truck has just about any part you will ever need, from dash pads to
fuel tanks to new gauge cluster modules to body panels.