ok can someone just be straight up. ive heard from so many different things from different people and mechanics about my check engine light after my test pipe. can some one please clear this up for me. what really happens from a check light from an O2 sensor

 

The ECU wants to see some activity from the primary sensor to allow it to trim the fuel ratio in closed loop. The secondary measures catalyst efficiency. On newer vintage vehicles the ECU actually will "test" the sensor by altering the fuel delivery every so often.

Sometimes the O2 sensor trouble code is only an indicator of some other problem. (Don't shoot the messenger)

 

the o2 sensor detects lean and rich mixtures. your car never runs at a perfect air/fuel mixture, its either rich or lean and the computer compensates one another with more or less fuel. all the sensors are complete circuits and when a sensor goes bad it creates a short and thats how you get a MIL on you cluster.