I'm having a problem with my truck running rich, I can smell unburned gas sometimes coming out the back of it. I'm going to test my o2 sensor, but I was wondering, what is the actual point to the o2 sensor? I know it is supposed to register whether the fuel mixture is rich or lean, but doesn't the throttle position sensor and mass airflow sensor tell the computer how much fuel the engine needs?

 

The computer uses the information from the oxygen sensor to adjust the injector pulse the throttle position sensor and airflow meter tells it how much air is entering engine. It constantly adjusts to maintain 14.7 to 1 air to fuel ratio.

 

TPS tells the ECU that the engine will change in excelleration or decelleration and idle.
AFM lets the ECU know Air temp and what it thinks the density of air is going into the engine.
The O2 sensor is the stool pigeon telling the ECU that the AFM is lying and to compensate,LOL.