When an RTP transaction is unable to process, it can failover to ACH if configured to do so. If it does, and still fails, there are additional codes that are returned.

Reason CodeDescriptionReason CodeError DescriptionBLKDPayment has been blockedAC02Debtor account Is InvalidAC03Creditor account Is InvalidAC04Account closedAC06Account is blockedAC07Creditor account closedAC10Debtor account currency is invalid or missingAC11Creditor account currency is invalid or missingAC13Debtor account type missing or invalidAC14Creditor account type missing or invalidAG01Transaction is forbidden on this type of accountAG03Transaction type is not supported/authorized on this accountAGNTIncorrect AgentAM02Specific transaction/message amount is greater than allowed maximumAM04Amount of funds available to cover specified message amount is insufficientAM09Amount received is not the amount agreed or expectedAM11Transaction currency is invalid or missingAM12Amount is invalid or missingAM13Transaction amount exceeds limits set by clearing systemAM14Transaction Amount exceeds limits agreed between bank and clientBE04Specification of creditor's address, which is required for payment, is missing / not correctBE06End customer specified is not known at associated Sort/National Bank Code or no longer exist in the booksBE07Specification of debtor's address, which is required for payment, is missing / not correctBE10Debtor country code is missing or invalidBE11Creditor country code is missing or invalidBE13Country code of debtor’s residence is missing or InvalidBE14Country code of creditor's residence is missing or InvalidBE16Debtor identification code missing or invalidBE17Creditor identification code missing or invalidDS24Waiting time expired due to incomplete orderDT04Future date is not supportedDUPLPayment is a duplicate of another paymentDS0HSigner is not allowed to sign for this accountFF02Syntax error reason is provided as narrative information in the additional reason informationFF03Invalid Payment Type InformationFF08End to End Id is missing or invalidMD07End customer is deceasedNARRReason is provided as narrative information in the additional reason informationRC01Bank identifier code specified in the message has an incorrect formatRC02Bank Identified is invalid or missingRC03Debtor FI identifier is invalid or missingRC04Creditor FI identifier is invalid or missingTM01Invalid Cut Off TimeTK01Invalid TokenTK02Sender Token Not FoundTK03Receiver Token Not FoundTK04Token ExpiredTK05Token Found with Counterparty MismatchTK06Token Found with Value Limit Rule ViolationTK07Single Use Token Already UsedTK08Token SuspendedNOATReceiving Customer Account does not support/accept this message type