|Side Quest: Fix the mine's air purifier|
|Speak to Zaius.|
|Travel to New Reno and speak to Renesco about the part.|
|Go back to Broken Hills and speak to Zaius.|
|Repair the air purifier.|
|Report back to Zaius.|
|Reward: 1000 XP, a combat shotgun or 500 dollars|
Talk to Zaius inside the house outside the Broken Hills mine's entrance. He will tell the Chosen One the mine needs parts for the air purifier and asks him to go to New Reno to pick them up. He can either take the reward for money, or decline and receive a combat shotgun when finishing the quest.
Return to Zaius and he will ask the Chosen One to enter the mine to fix the purifier. The mine is filled with radon gas which will damage the Chosen One and his party for 10-15 armor-bypassing damage every couple of seconds. Wearing power armor will negate the damage. Use the parts on the clanking machine in the back (but watch out for the deathclaw!) for 1000 XP.
Return to Zaius and he will give the Chosen One a City-Killer combat shotgun or $500 if he asked for a reward earlier.
Behind the scenes
The password that Zaius gives the Chosen One during this quest is "The canary's kicked the bucket." This is a reference to a practice in which miners would bring a caged canary into a mine with them to detect poisonous gas leaks in the mine. Because a canary's body mass is much smaller than a human's, the canary would pass out or die a significant amount of time before a human would, giving the miners an early warning to escape the mine before it was too late. Hence, the "canary's kicked the bucket" because the mine was filled with radon gas.