It depends why you are in coach. If it is because it is one of the Economy Class only flights (AA3000-5999, AY domestic, LA domestic or EI domestic) then yes you book into Y. But if it is simple lack of availability then you have to book into D for business, or if that is not available then L for economy.

Details here: http://www.hardlink.com/~markdu/OWFi...kingCodes.html from which:

"29N . * NORMAL FARE BOOKING CODES MAY NOT BE USED ON
30N . RTW OR CIRCLE PACIFIC FLAT RATE FARES."

So you have to use the codes in the table there.