Yeah, defeated by yellow. *g* Although I think they might have retconned that? *checks Wikipedia*

In Green Lantern: Rebirth it is revealed that Jordan was under the influence of a creature known as Parallax when he turned renegade. Parallax was a creature of pure fear that had been imprisoned in the Central Power Battery by the Guardians of the Universe in the distant past. Imprisonment had rendered the creature dormant and it was eventually forgotten, becoming known merely as the "yellow impurity" in the power rings.

[....] Kyle Rayner was chosen to wield the last ring because he knew fear, and Parallax had been released from the Central Power Battery. Ganthet knew this and chose Kyle because his experiences dealing with fear enabled him to resist Parallax. Because Parallax is fear, and yellow, none of the other Green Lanterns, including Hal, could harm Parallax and, therefore, came under his control. Kyle taught them to feel and overcome fear so they could defeat Parallax and incarcerate him in the Central Power Battery once again.

... This is pretty much standard comics logic. XD