ACH Return Codes Explained: R01–R85 and How to Handle Them in Production Understanding ACH Return Codes: A Developer's Guide When an ACH transaction fails, you don't get a generic "payment declined" message. Instead, the National Automated Clearing House Association (Nacha) returns a specific two-character code that tells you exactly why the transaction was rejected. Understanding these codes—and building logic around them—is essential for any developer managing payouts, disbur...