Check engine light has no code

[gruntzz]

My check engine light came on the other day. I ran a code reader on it and it posted no error code. I tried running the clear code option (reset the light) and it responded with an error most likely resulted from no codes stored. It has been running great. Matter of fact it came on the day after having service performed (Subaru) for overheating. Is there anything that can cause a CEL without posting a code or even being able to clear with the code reader. I don't know the brand or model it is one of the $90 ones from Harbor Freight, and only a couple months old.