Hank:
Check Engine lights are a funny thing. They can be a big deal or no deal. I've been driving my Boxster with a CEL for 10 yrs & 60,000 miles. Same with my Vette & my Honda Odyssey for 12 years and over a 100k miles. I've forgotten why the CEL is even on.
Getting back you, has anyone told you why the CEL is on?
A simple code read would let them know what's causing it and then it would be your call whether to address it or not. The reason the CEL was off last time you picked the car up is because your guy must have read the code and then had his code reader clear the code. The issue is still there so it's only a matter of time before the CEL comes on again.
I clear my codes each time my cars are do for NJ State inspection. I bring it to the MVC and pass each time and within a short time the CEL is back on.
Are you sure no one told you what's causing the CEL to come on? Evidently no one thinks it's a serious issue.
If you are located anywhere near central Jersey, the East Brunswick area and you want to have possibly the best Miata guy in NJ/Pa look at your car you should get it to Dave Deerson @ T&Z Auto in South River. He can take apart and rebuild a Miata with his eyes closed. He's a one man shop so be patient if he does not answer the phone or respond to your voice message in minutes. 732-257-2664
Good Luck
Morris West