There are three oxygen supplies in CSS: 1) If your aggregate written score is atleast 600 excluding failed paper then you are eligible to appear for the interview. 2) If you have opted 200 marks subject which requires two papers. If you fail in one paper but that deficit is met from the second paper to reach minimum passing marks threshold then you are pass in that subject. 3) There is a bunch of three papers in General Knowledge. If you fail in two papers and your third paper fills the gape of aggregate score of 120 then you are pass in GK. In 2nd and 3rd life lines, score of failing paper is also counted in your merit whereas in 1st oxygen line, marks of failing paper are not counted in your merit.
Courtesy of Senior member.