You can get your error codes read for free at some auto parts stores.
When you have the code, you have to understand it, usually do some more troubleshooting under the hood and then know what and how to replace/repair. Most of this is beyond the ability of the old lady who has trouble connecting her printer.
I see it as a good thing to know that something is wrong, but it does take a professional to fix it. It is no different from a strange noise under the hood. A dishonest mechanic could take advantage of an innocent client, with or without a CEL.