Hyundai Repair: Check engine light, sonata 4, hyundai sonata

Question
HT,

I have a 1999 Hyundai Sonata 4 cyl, automatic.  My check engine light comes on during trips over 50 miles.  The next morning I start my vehicle and the check light is gone.  This only happens on long trips.  My temp gauge is within normal range while travelling.  I cannot isolate this problem because of its intermittent nautre. What could it be.  Bad sensor? Loose connection? Please help!

Thanks,

Shane  

Answer
Without a diagnostic trouble code, the possibilities are about 36 pages of problem types.  

The first step is to have your engine and transmission diagnostic trouble codes read.  The codes will give you information about the nature of the problem.  AutoZone has advertised they'll check diagnostic trouble codes for free, but it's not clear whether they can read the transmission codes.