The fault locator was a component of the TARDIS, 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 it was determined that one of the fluid links had run out of mercury. (DW: The Daleks)
Shortly afterwards, when the Fast Return Switch became stuck, the fault locator was unable to indicate the source of the problem, as the switch was not broken, merely stuck. In an attempt 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 once every fifteen seconds, indicating that the ship would be destroyed in fifteen minutes. (DW: The Edge of Destruction)
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. (DW: 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. (ST: The Three Paths)