Check engine please advise Code P0100, P0325

[sebastianpiz]

hi,my engine sputters and have hesitation on freeway or city driving. Check Engine light is on. I used an OBD Scanner. Code P0100- MAF Circuit or AUF Circuit. came out. then another code came out P0325-Knock Sensor, Bank 1 or Sensor 1. My question is I know the MAF Circuit but what is a AUF Circuit. MAF Circuit is $225 but before i replace it how can i be sure that it is not the AUF Circuit. How do you test if the MAF or the AUF is at fault? What is an AUF? Please advise.

thanks,