How to know if I need to replace my o2 sensor?

A check engine light indicates there is a diagnostic trouble code set. So, there is definitely a code, but perhaps you don't have a good way of retrieving the code. And then - you'll have to interpret the code and test for the root cause of the code next. So, some assistance from a tech near you would be helpful. www.repairpal.com/repair-shops. To answer your questions about the "O2 sensor" -- They need to be replaced if they are not working properly. This can be verified by hands on testing with a diagnostic tool that can monitor component operation..