The Viking project went through an extended series of extended missions. Due to shuttle related budget problems, and the indifferrence of the Carter administration, there were recurring threats to kill the missions to save money. The landers were eventually placed on entirely automatic repetitive observation cycles, while orbiter operations were carried out with "stripped" operations teams, accepting higher risk than would normally be acceptible.

Both orbiters had problems with "sticky" cold-gas <nitrogen> attitude control jets: the pulsed valves' seats got gunk or grit and didn't quite seal every time. Orbiter 2 died from attitude gas depletion after 2 years, Orbiter 1 lasted 4 years before it's gas supply ran out.

Lander 2 had it's direct-to-earth radio transmitter fail, and could only communicate through the orbiters by relay. However, it died of battery failure a few weeks before Orbiter 1 died and it would have lost communications capability anyway. It worked about 4 years on Mars.

Lander 1 lasted about 6 1/3 years before it started to show signs of battery failure. An ad-hoc operations team worked up a battery-conditioning sequence to try to keep it running longer. Unfortunately, when the commands were uplinked, they apparently overwrote <oops> software that programmed the antenna pointing for the direct-to-earth transmissions. Despite attempts to predict times when the antenna would be pointing at Earth during mis-programmed gyrations and repeated tries to uplink a patch, Viking Lander 1 was never heard from again. (I suspect even if there was no booboo, the battery conditioning would have only bought a little more time.)

Note that the landers were powered by radio-isotope thermoelectric generators, but the output from the generators was low, maybe 70 watts each (I don't have the numbers at hand) at the start of the mission. Typical lander activities required a few hundred watts to run transmitter, tape recorder, instruments, etc., so the batteries had to supply the extra watts. Battery failure left the landers "alive" but they'd "safe" themselves and shut down operations when they tried to use enough power to do anything useful.