Hyundai Repair: check engine light, transmission control module, transmission control module tcm

Question
2000 Elantra.  Periodically the sevice engine soon light appears.  Then it goes away for a while.  Faulty gas cap?  Any ideas?

Answer
A faulty gas cap is a possibility, but there are many, many possibilities.  You'll need to start by reading the diagnostic trouble codes in the engine control module (ECM) and transmission control module (TCM).  This will tell us the nature of the problem.  AutoZone and AAMCO have advertised they'll read your trouble codes for free.  Follow-up with the trouble codes here, and I'll advise you on how to proceed with diagnosis and/or repair.