the "check engine" light keeps coming on for no particular reason, and the system has been reset. any ideas?

Three days ago, the little symbol that looks like an engine came on with a loud ding
accompanying the signal.  I had no idea what it was about, so went to my garage
where a mechanic checked it out with his computer analyzer.  He said something
about the fuel mixture which I did not understand, but today I took it back to the dealer
(car was bought for me as a present the last Sunday of January, 2014) and the
mechanic there told me he found that there was kind of a computer "glitch" which had
hung up the reading on my garage man's computer.  The dealer mechanic said he reset
the computer and felt that should clear up this situation.  Well, it did not...the same
signal, ding, and little engine symbol came on this afternoon (the computer was reset
this morning.)