Yes, this is what can be called an incomplete sentence. I think it can still be a sentence though. It could be an answer to the question: "Which car is yours?".
But I see how it can throw somebody off. To make it more pronounced, I can insert an "egy":
"A kék háztól a második egy autó". - "The second (object) from the house is a car".
OK. Maybe it is a difference in terminology. But, I think, Hungarian grammar looks at it this way: (almost) everything is a sentence, but some sentences are incomplete. Something is missing from them but they are still sentences. So we don't have sentences and "whatshouldicallems", but instead we have sentences, some of which are incomplete.