So I have a brand new 2014 v6 premium mustang, right now has roughly over 3,000 miles. Only thing done to it is a K&N filter and an sr throttle body spacer and the check engine light comes on a couple days after I put the spacer on. Everything else is stock. What could make a brand new cars check engine light come on?

 

Whenever I have encountered a change in behavior, whether of a computer system or a car or whatever, my first question has been what has changed. It seems to me that you have already answered your question. Your check engine light came on after you changed the throttle body. I assume you did not install the K&N filter and the throttle body at the same time. If that is correct and you had no problems after installing the K&N, try putting the original throttle body back on and see what happens.

 

Remove the throttle body spacer. If anything you did required an aftermarket tune, that could be an issue too.