Was the customer charged?

Usually not, because the issuer never returned an approval. But a 91 caused by a timeout rather than an outright outage can leave the issuer holding an authorisation it approved after the network gave up. Where your acquirer's specification calls for a timeout reversal, send one — that is exactly what it is for.

How should I retry a 91?

With exponential backoff and a cap, and with the same trace or idempotency reference, so a duplicate cannot be created if the first attempt did land. Retrying immediately during an issuer outage just adds load to something already failing.

ISO 8583 response code: 91

The issuer's authorisation system could not be reached. Nothing is wrong with the card or your message — the decision could not be obtained, so the transaction failed by default.

Issuer or switch is inoperativeRetryable

The same request can succeed later. Retry with backoff.

What it means

A 91 is a routing and availability failure. The acquirer or network could not reach the issuer within its timeout, so it returned this rather than an authorisation decision. Because the issuer never saw the transaction, no funds were held and nothing needs reversing in the normal case — but this is also exactly the situation where a stand-in authorisation or a timeout reversal may be in play, so it is worth confirming the transaction's final state rather than assuming.

Common causes

The issuer's host is down or in a maintenance window.
A network or switch failure between the acquirer and the issuer.
The issuer's response arrived after the network's timeout, so the transaction was abandoned even though the issuer may have processed it.
A scheduled outage for a specific BIN range, affecting only cards from one bank.

How to fix it

Retry, with backoff. This is one of the genuinely transient codes, and a retry a short while later often succeeds.
Do not treat it as a decline in your metrics. Counting availability failures as declines will make a bank outage look like a fraud problem.
Send a reversal if your acquirer's specification requires one on timeout. An issuer that responded after the timeout may be holding funds you did not capture.
Check whether the failures cluster on one BIN range. If they do, it is one bank's outage rather than anything on your side.

Tools that help

ISO 8583 message parser

Frequently asked questions

Was the customer charged?: Usually not, because the issuer never returned an approval. But a 91 caused by a timeout rather than an outright outage can leave the issuer holding an authorisation it approved after the network gave up. Where your acquirer's specification calls for a timeout reversal, send one — that is exactly what it is for.
How should I retry a 91?: With exponential backoff and a cap, and with the same trace or idempotency reference, so a duplicate cannot be created if the first attempt did land. Retrying immediately during an issuer outage just adds load to something already failing.