Dear Customer (name blocked for privacy),
Unfortunately, the Costco return policy does state items must be returned to the store within 90 days of purchase (http://www.costco.com/Service/FeaturePage.aspx?ProductNo=11217032&cm_re=1-_-Right_Nav-_-Concierge%20Services) However, if the product was defective when you purchased it, you may be able to have the manufacturer repair or replace it under warranty as long as you contact them within the time period they state for such things. The instructions or manuals that came with the product should have a contact number or address for the manufacturer.
Also, because you bought the product through Costco, you may want to contact their Concierge Service (listed on the above website) to see if they can help you get a replacement or repair.
Lastly, if none of these options work out for you, you may need to contact the IL Attorney General's office of consumer protection to see if they can help you with this situation - http://www.ag.state.il.us/consumers/index.html
PLEASE NOTE: Responses here are for information/education only, NOT legal advice and do not form attorney-client relationship! Only licensed attorneys you hire in your state can provide legal advice.