What does a 503 error mean? Thaabit Mollagee March 09, 2016 08:47 Updated Follow This is a timeout error receive by merchants on 3D Secure when attempting to do a 3D Secure lookup. Related articles Why do I receive an error, transaction index required? I receive an error “merchant not participating", what does this mean? MyGate API Response Codes Comments 0 comments Please sign in to leave a comment.