No.
Consciousness is not epiphenomal
The Argument from Deficits
(okay, this is not the most perspicacious graphic for this argument, but I liked the picture of the dog)
One common and successful strategy for understanding the function of a trait is to study subjects that are deficient in some regard. For example, some patients with brain damage report no awareness of phenomena in half their visual field. However, they can, in forced-choice situations, perform significantly better than chance at verbally "guessing" properties of stimuli present in that hemifield and at pointing to objects there. They have, in some sense, "blindsight". (See, e.g., Weiskrantz (1997).) Milner and Goodale (1995) content that there are two visual systems in the brain, one for action and one for judgements. It is surprising, almost paradoxical, that blindsight subjects can "see without seeing", so to speak, but it is perhaps even more surprising that their abilities to initiate action are so impaired. Simply put, they do not think they are receiving any perceptual information, and must be persuaded to act. In normals, the two pathways are integrated and, hence, conscious awareness is hypothesised to be necessary to engaging in deliberate action. (Other clinical evidence of this sort involves subjects with prosopagnosia and epilepsy (Block (1997), Hardcastle (1995)). See also Kosslyn and Sussman (1995), and Grossberg (1999).) Ergo, consciousness is not epiphenomenal.
Response:
Yes, the conscious experiences of blindsighters and normals differ, as do their capacities. But we cannot infer from this fact that the differences in consciousness are causally efficacious. Reminiscent of the reply to the Argument from Evolution, above, it is consistent with the data that the conscious aspects of some states are unavoidable concomitants of the causally efficacious properties of those states.
Simply put, exploring deficits is provocative and can help us learn a great deal about human capacities, but, in this case, it cannot help us establish that consciousness is causally efficacious, given that there are a number of potentially relevant differences between blindsighters and normals.