Online Center-Set Default Bank
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The Online Center account selection defaults are: If you have an online-enabled account open when you open the Online Center; the Online Center will open to that account. If you do not have an online-enabled account open when you open the Online Center; the Online Center opens to the first listed online-enabled account available at the financial institution that last had an account displayed in the Online Center.
I concur with Chris_QPW's comment and suggest creating bill pay transactions in the account register. It's not necessary to use the Online Center to pay online bills. You can easily create Online payment transactions directly in the appropriate account register, by selecting "Send Online Payment" in the Check# dropdown and selecting the appropriate Online Payee in the Payee dropdown.