Fix the mine's air purifier

Fix the mine's air purifier is a side quest in Fallout 2. To complete it, the Chosen One must retrieve the mine air purifier parts from New Reno, and then go into the mine itself to fix the purifier.

Detailed walkthrough
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 them to go to New Reno to pick them up. They can either take the reward for money, or decline and receive a combat shotgun when finishing the quest.

Talk to Renesco the Rocketman in New Reno. Tell him Zaius' password. If the Chosen One doesn't, they need to pay 200 NCR dollars for the parts ($100 with a Barter or Speech check).

Return to Zaius and he will ask the Chosen One to enter the mine to fix the purifier. The mine is filled with gas which will damage the Chosen One and their party for 10-15 armor-bypassing damage every couple of seconds. Wearing power armor will negate the damage. Use the parts on the machine in the back for 1000 XP.

Rewards
Return to Zaius and he will give the Chosen One a combat shotgun or $500 if they 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 bring a caged canary into a mine with them to detect gas build up.