It depends entirely on the circumstances.
The operator of an emergency vehicle is obliged to slow and check the intersection for cross traffic while operating as an emergency vehicle with lights and siren. If the operator of the vehicle he strikes has done something that contributes to the accident, he could be held liable. If the operator of the emergency vehicle has failed to slow and check traffic, he could be at fault.
I was once T-boned by a crossing vehicle while running Code 3 because the other driver was looking in his rear view mirror to see where the siren was coming from. (He actually told me this when I crawled out the passenger side of the patrol car to check on him.) The city cops who investigated the accident cited him for failure to yield to an emergency vehicle. The muni judge who heard the case dismissed the charges.
