Mattvoza

i am trying to figure out what is wrong with my 2000 gxe that i just got. let me start with i didn't know anything was wrong with the car because the check engine light is burned out. i didn't know there was something wrong with the car till we took it to inspection later that week(i didn't think anything was wrong so we bought the car with no inspection sticker). it says i need to get a new cat i hooked up my odb2 meter and saw that the SHRTFTB2S2 was holding at 99.2 (idk if that is normal). so from other peoples experience should i try the o2 sensor first or the cat.

now the second question is why is my odb1s2 sensor reading 0 volts and my stftb1s2 is reading n/a. is this normal?