Upload & Sell: On
...on the front there is a solid red light
The red light on the front should flash when the 430EX is in Slave mode, and it should be off when mounted on the camera. It could also be the AF Assist Beam emitter, which should only be on when the shutter button is half-pressed. If it's on constantly, then I think you have a logic error or an internal short.
If taking out and then replacing the batteries doesn't fix it, I think it's a repair/replace situation.