What is Auth Code 51, declined?

A credit card processing response of Auth Code 51, is a decline for insufficient funds, the credit limit has been exceeded. What happens when the customer says, “there’s nothing wrong my Visa card, put it through again”? If put through again without a voice authorization, the merchant is at risk for chargeback of funds for invalid authorization.

Visa Product and Service Rules, 8.4.1.3 Original Credit Transactions – Prohibition against Clearing a Declined Transaction

An Originating Member must not send an Original Credit Clearing Transaction if it received a Decline Response to the corresponding Authorization Request.

Further information at page PSR-564, 11.1.16 Chargeback Reason Code 71 – Declined Authorization. NEW. Effective for Transactions completed on or after 15 April 2016,
A Transaction for which Authorization was obtained after a Decline Response
was received for the same purchase. This does not include an Authorization
Request that received a Pickup Response 04, 07, 41, or 43 or was submitted
more than 12 hours after the submission of the first Authorization Request.

This period is known as the black hole or dark period. For the first 12 hours after a decline, merchants should not attempt to process the same retail transaction. The reality is a consumer could simply walk away and go back to another cashier and try again. Some cloud based payment gateways will enable merchants to choose to prohibit multiple attempts in the black hole period.

Disclaimer: The rules of card acceptance are very complex. Merchants should read the manual for complete details regarding card acceptance for your business type.