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The fault locator was a component of the TARDIS. It was located in the computer banks lining the console room wall. It was used to detect the source of malfunctions on the ship.
When the TARDIS failed to dematerialise from Skaro, Susan checked the fault locator and determined that one of the fluid links had run out of mercury. (TV: "The Dead Planet")
After the Doctor disables the antennae in the Dalek City, Susan warns that the Daleks must have their own fault locator somewhere, before both are ambushed by Daleks. (TV:"The Ordeal")
Shortly afterwards, when the fast return switch became stuck, the fault locator could not indicate the source of the problem as the switch was not broken, merely stuck. To warn the crew of the danger, the TARDIS had the fault locator completely lit up, indicating that every system was breaking down. The lights on the locator flashed every fifteen seconds, indicating that the ship would be destroyed in fifteen minutes. (TV: "The Brink of Disaster")
Some time later, the fault locator registered faults just before the TARDIS doors opened in mid-flight; however, after the TARDIS materialised, the fault locator indicated that nothing was wrong. (TV: Planet of Giants) It was much later revealed that it had broken down at some point but claimed it was working perfectly, which made repairs to it almost impossible. (PROSE: The Three Paths)
When the TARDIS failed to change its location on Tiro, the Doctor checked the fault locator and determined that one of the klister valves had run out of magnatite. (PROSE: Terror on Tiro)
When the TARDIS warned the Seventh Doctor that it was in trouble, he first tried checking the fault locator, but it did not detect any malfunctions. He eventually realised that an alien creature was attempting to get into his ship. (PROSE: Cat's Cradle: Time's Crucible)
By the time that Susan and her son Alex joined the Eighth Doctor and Lucie in the TARDIS for Christmas, the fault locator had itself developed a fault. (AUDIO: Relative Dimensions)
Behind the scenes
During filming of The Edge of Destruction, William Hartnell accidentally said "fornicator" instead of "fault locator". This did not make it into the finished product, but the story lasted long enough to be featured in the 2013 docu-drama An Adventure in Space and Time.