I don't believe that Jesus sinned, but I've heard it said by many theologians that He was not able to sin. He could not have yielded to Satan's temptations they say. I say that this would not be a true test of faithfulness to the Father. If he could not have sinned then He was still relying on His God nature to keep from sinning. This would not satisfy the need for someone to perfectly keep the law in order to pay the penalty of sin for others. He needed to be able to sin in order for his obedience to the Father to be actual obedience and not the imitations of humanity like an automaton.