Apple thinks I have a defective WIFI card????

I'm booked to take my 2013 Macbook Air in tomorrow as Apple thinks I have a defective network card. Could that be possible. I told them that if I set my network to as an extended network it is 3 times as slow than if I set it as a joined network. However, is it is set as a joined network my IOS devices run at a snais pace.

This is all odd. Say they do find there is a problem with the card are they even able to open up a Macbook error to correct the problem.

Any thoughts would be appreciated.