If an error condition is intermittent the CEL may not always come on, but it would leave a pending error code in memory. If you drive the car enough to complete what is called a "drive cycle" which requires multiple driving conditions, and no error is registered, it will clear the prior error code. Autozone can clear the error codes after reading them, but if your state allows the on board ODBII emissions check (rather than tailpipe) it would not pass inspection without the drive cycle being completed.