Your question triggered a memory that the Viking spacecraft were all shut down a long time ago. According to this source, the Viking 2 lander was turned off on April 11, 1980 as its batteries were failing. This source states that the Viking 1 lander operated until November 13, 1982 when it contact was inadvertantly lost due to a faulty command.

As for the orbiters, this source states that the Viking 1 orbiter operations were terminated on August 17, 1980. The Viking 2 Orbiter was turned off on July 25, 1978. One of the biggest constraints on Orbiter life was probably propellant. They probably ran out of propellant to perform momentum wheel (reaction control wheel) unloads. Without the ability to unload momentum from the wheels, you can't maintain antenna pointing necessary for communications.