[Don't tell 'Ma]

The dreaded "Check Engine Soon" light has lit on my 2000 Mustang V-6 with 95K miles. So, I buy an OBD II code reader and find that the fault is code P0411, "Secondary Air Injection System Incorrect Flow Detected". However, it is running fine...

The question, then, is what the heck does that mean? What is the true culprit? Is there a sensor to replace? Will the light return?

Any help is appreciated...