Today I started receiving errors with 1st data services. Their system was returning this error.
x_allow_partial_auth : x_allow_partial_auth too long. Maximum length is 4

A call to their support found that several zencart users had called with this problem. They advise either change 'x_allow_partial_Auth' => 'FALSE' to True, or comment the line entirely as they do not use it.
I commented it out from includes\modules\payment\authorizenet.php and it solved the problem.