1999 Mazda Miata Problem

I have driven Mazdas for 25 years, and am sharing these experiences in case they can help anyone.  I had two RX-7s and then bought a 2004 Miata a year ago.  The Miata really was brand new, a showroom car with minimal miles on it--truly mint, and a true dream come true.  
It ran perfectly but the 'check engine' light kept coming on.  The owner's manual lists four possible reasons:  1) fuel tank level being very low or approaching empty; 2) electrical system has a problem; 3) emission control system has a problem; and 4) fuel-filler cap is missing or not tightened securely.  
Given the car's brand new condition, and since it had a full tank of gas courtesy of the Mazda dealer, I could rule out 1 through 3. That left the fuel-filler (gas) cap.   I had driven the car minimally the first month, so the gas tank was still almost full when I took the car back to the Mazda dealer.  When I was instructed about the importance of making sure the gas cap was secured tightly, I said, "Interesting you should mention that, since I haven't touched it since I bought the car, so if it isn't properly affixed, guess who did it?" and laughed.  I asked if it was a "software" glitch between the updated software used in the shop's system checking equipment and the fact my car was three years old. That resulted in a stunned look.  I barely had finished a small cup of coffee when the manager came to tell me everything was fine.  The answer about what was the problem was vague, at best.  Zoom-Zoom, off I went, and the 'check engine' light hasn't appeared since then.  I do find the gas cap difficult to manage, especially in the winter.  
The Bose CD player died last week, but that's a different problem!  G-R-O-A-N.  
I am a huge fan of Mazda.  Of course the RX-7s had the rotary engine and the Miata does not, but there are similarities.  These cars dislike condensation.  I am in the northeast where winter temperatures reach minus ten or lower, and where summers are very humid.  
I went through the spark plug changes and all else with the first RX-7 when it seemed to have a problem misfiring, or not making a solid connection.  It had a manual choke, so starting it was easy, but after it was warmed up and ready for takeoff, it would perform as if it had a short or dirty plugs or something.  
I was religious about having the oil changed every 2,500 to 3,000 miles, very important. When it seemed the car was misfiring or just not connecting on some level, and everything checked out perfectly mechanically and electrically, I decided to try two things.  First, I only used the best gas.  Expensive, yes, but the car performed better and got better mileage.  Second, I always kept a bit of dry gas in the tank, all year round.  When I had the second RX-7,  I lived near a lake and there always was heavy mist and condensation.  If I kept dry gas in the RX-7s, they didn't have a problem.  If I forgot, the response was chugga-chugga-chugga, cough, spit, hesitate, misfire, and generally behaving like a misfit.      I should stress I ONLY am talking about dry gas, sold everywhere in the northeast during the winter, even drug stores and supermarkets.  There are tons of additives on the market that claim to do this and that and I never have used any of them.  
We love these cars because we feel one with them.  They are so beautifully engineered, we notice every nuance, every single thing that seems a tiny bit off.  The best advice I can give anyone about these great cars is to trust that knowledge, and to be persistent if you know something is not quite right, no matter what someone tells you to the contrary.  I wish everyone as much joy as my Mazdas have brought me . . . now, on to that Bose problem . . . how I landed on this web site in the first place.

You could have changed the plugs and wires yourself for under 50.00