It depends on what type of financial aid you are awarded.  
Federal aid (i.e., Pell Grant, FSEOG, Subsidized and Unsubsidized Direct Loans, Perkins Loans, Parent PLUS Loans, Grad PLUS Loans) 
cannot be applied to a prior year's past due balance, per federal regulations.
ASU institutional aid (i.e., University Grant, FA Trust Grant, FA Program Fee Grant) will apply to your prior year's past due balance 
only after your current semester's charges are paid in full. Keep in mind that your current semester's financial aid does not disburse any sooner than 
eight days before the semester begins.  
Therefore, it is vital that when you receive a financial aid refund, that you check 
My ASU, under the 
Finances tab, in the 
Account Charges box to see if your past due balance from a prior year was paid. In some cases (i.e., awarded federal aid only) you could receive a financial aid refund of your current semester's leftover financial aid while still having a past due balance from a prior year. In that situation, you can use your current semester's financial aid refund to make a payment on your prior year's past due balance.
Related Article: Can I use a private loan to pay my past due tuition?Need additional help?  Access 24/7 live chat, create a case from 
My ASU Service Center or call 855-278-5080.