2002 Chevy Blazer CO2 sensors

2002 Chevy Blazer 6 cyl Four Wheel Drive

hi guys! My engine light has been on for a while and I recently had someone test it to see why it was coming on. I was told it was 2, CO2 sensors that are going off. Two days later my fuel pump died. Well once that was repaired I thought things would be ok but unfortunately the light came back on. So I don't get raped at the garage can you please help me and tell me what my problem might be and how costly it might run me? Thank you so much-- mary jo