S3 Troubleshooting playbook

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Lesson S3: From a symptom (response code, HSM error, KCV mismatch, MAC failure) to the root cause.

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The opening of the lesson

When payments fail, the symptom is usually far from the cause:

a PIN "fails" at the issuer because the terminal took the wrong PAN digits;
a MAC fails because one side pads differently;
cardholders complain of double charges because a reversal pointed to the wrong STAN.

Troubleshooting is the skill of walking back from the symptom to the cause, checking the cheapest and most likely things first.

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