The elevator refers to the elevator in the apartment building where Sheldon, Leonard, and Penny live. It has been out of order since 2003, four years before the series began, as told in “The Staircase Implementation “( S3E22) although Leonard once said two years previously in “The Nerdvana Annihilation “( S1E14 ). It has been broken ever since Leonard tested out an experimental rocket fuel formula which began to explode, with Sheldon saving Leonard’s life after he tried to take the fuel outside by going down the elevator, but was stopped by Sheldon closing the elevator after taking Leonard out, in “The Staircase Implementation “.
Ever since Penny moved in, she has had to take the stairs instead of using the broken elevator (“The Staircase Implementation ” ). Many people new to the building have tried to go in the elevator, before realizing it is broken, such as Zack Johnson (“The Lunar Excitation “, S3E23 ), Beverly Hofstadter (“The Maternal Capacitance “, S2E15) and Ramona Nowitzki (“The Cooper-Nowitzki Theorem “, S2E06 ), although she was merely wondering why it hadn’t been fixed. Howard once tried to fix the elevator, but upon inspection gave up instantly (“The Nerdvana Annihilation ” ).
When Sheldon’s mother, Mary, asks when his landowner is going to get the elevator fixed, he replies that he doesn’t know; however, he wants to turn the elevator shaft into a missile silo in order to strike Burbank before they get them (“The Rhinitis Revelation “, S5E06 ).
In “The Maternal Capacitance ,”,NNP, ” Penny meets Leonard’s mother when she is staring at the elevator door trying to understand the implications of the broken elevator.