What causes Decline: MP

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MP error is thrown when the card read is failing due to the "Magnaprint" check that occurs on all Magtek encrypted reader transactions. This check is validating that the card information on the magnetic stripe of an individual card matches up with what has been processed before with that same physical cards magnetic signature. This is built in to prevent the copying of cards. It is an issue that you might run into when dealing with test cards as they are often using the same card number copied onto different physical cards. An easy way to test if this is the issue is to run a unique card for $0.01 and see if it processes all the way through to an approval. If this does work than the issue is with the test card itself and not the reader or the integration.