Check Engine light came on in my car a few months ago and then went off. It has been back on for the past few weeks but don't notice anything wrong with the car. The only thing is the defrost seems to be off when it is humid causing car to fog up inside more than usual. Took car to AutoZone and guy said it was small evaporation leak causing knock sensor to go off? Anyone had this issue?

What was the actual Diagnostic Trouble Code (DTC)?
The defroster won't set a code that can cause the light to come on, nor will an evaportor leak cause the knock sensor to go askew.
You may need to have another shop look into what is causing the light to come on.

AutoZone is handy to get the codes on your car, but are very often not trained to properly diagnose the problem. The evaporation system keeps gase fumes from your gas tank from going into outside air. The knock sensor is used to tell the computer what to time the motor at. They are not related. I could give you a better awnser if I knew excatly what the codes are. If your defrost isn't working properly, it may have to do with your a/c not working properly.

Thanks, guys! My car did fail inspection for emissions. And the codes that came up for my 1999 Nissan Altima were p0325 and p1440. Car seems to be running fine. Does that help provide more info.

The P0325 is for a fault with the Knock Sensor Circuit.
The P1440 is for a fault with the Evap Emissions System.
Neither will cause noticeable drivability symptoms.
Both will need diagnosed by a tech with the skills and a scan tool.